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SI3 
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INSTRUCTIONS 


FOR 


OBSERVING  THE  TRANSIT  OF  VENUS, 


DECEMBER    0,    1882, 


PREPARED   BY 


THE  COMMISSION  AUTHORIZED  BY  CONGRESS, 


AND 


PRINTED  FOR  THE  USE  OF  THE  OBSERVING  PARTIES  BY  AUTHORITY 
OF  THE  HON.  SECRETARY  OF  THE  NAVY. 


DOCIJ 

oc 


WENT 


WASHINGTON: 

GOVERNMENT   PRINTING  OFFICE. 
1882. 


INSTRUCTIONS 


FOK 


OBSERVING  THE  TRANSIT  OF  VENUS, 


K    (5 ,    1  8  8  2  , 


PREPARED   BY 


THE  COMMISSION  AUTHORIZED  BY  CONGRESS, 


PRINTED  FOR  THE  USE  OF  THE  OBSERVING  1'ARTIES  BY  AUTHORITY 
OF  THE  HON.  SECRETARY  OF  THE  NAVY. 


WASHINGTON: 

GOVERNMENT     PRINTING     OFFICE. 

1882. 


[GENERAL  ORDEK.] 

NAVY  DEPARTMENT,  Washington,  August  u,  1882.. 

The  parties  organized  by  the  Secretary  of  the  Navy  to  observe  the  Transit  of 
Venus  in  December,  1882,  under  the  authority  of  an  act  of  Congress  approved  August 
7,  1882,  are  organizations  invested  with  naval  character,  and  subject  to  naval  rules, 
regulations,  and  discipline.  The  command  of  each  party  is  assigned  to  the  Chief 
Astronomer,  to  whose  authority  all  others  will  be  obedient. 

The  following  is  the  order  of  rank  and  authority  in  each  party,  viz: 

1.  Chief  Astronomer. 

2.  Assistant  Astronomer. 

3.  Chief  Photographer. 

4.  Assistant  Photographer. 

This  order  will  be  respected  and  followed  as  the  order  of  rank  and  succession  in 
each  party  under  all  circumstances,  including  the  contingencies  growing  out  of  any 
separation  of  the  p^rty,  or  the  happening  of  any  vacancy. 

The  party  destined  for  the  coast  of  Patagonia,  about  to  embark  in  the  U.  S. 
steamer  Brooklyn,  will  be  personally  subject  to  the  authority  of  the  commanding  offi- 
cer while  on  board  that  ship,  and  to  its  discipline,  police  authorities,  and  regulations. 

The  gentlemen  engaged  in  this  interesting  and  important  service  will  readily  and 
fully  understand  that  discipline,  harmony,  and  co-operation  are  essential  to  its  satisfac- 
tory performance  as  well  as  to  their  own  safety  and  convenience,  and  that  for  this 
reason  they  have  been  required  to  assume  the  obligations,  and  are  subjected  to  the  rules 

of  the  naval  service. 

WM.  E.  CHANDLER, 

Secretary  of  the  Navy. 


799 


TABLE    OF    CONTENTS. 


Order  of  the  Honorable  Secretary  of  the  Navy  respecting  these  instructions 7 

I. — Introductory  note 9 

II. — Selection  of  station 9 

III. — Laying  oft'  ground  and  setting  np  instruments 10 

IV. — Memoranda  respecting  the  photoheliograph 13 

V. — Adjustments  of  the,  photoheliograph    14 

VI.— The  heliostat 16 

VII. — The  exposing  slide 17 

V1IL— The  tube    17 

IX. — The  plate-holder  plumb  line 18 

X. — Batteries  and  electrical  connections 18 

XI. — The  measuring-rod    .    19 

XII. — Instrumental  errors  . 20 

XIII. — Fittings  of  the  photographic  house 21 

X IV. — Care  of  the  sensitive  emulsion 21 

XV. — Selecting  and  marking  glass 22 

XVI. — Cleaning  and  albumenizing  glass   .    ... 22 

XVII. — Coating  plates . . . . : 24 

XV111. — Exposure  of  plates  in  the  photoheliograph 26 

XIX. — The  development ., 27 

XX. — Fixing  and  varnishing  29 

XXI. — Blistering  and  lifting  of  the  film 29 

XXII. — Spots  and  other  defects   29 

XXIII. — Preparatory  practice 29 

XXIV. — Preparation  for  the  transit 30 

XXV. — Photographing  the  transit 31 

XXVI. — Development  and  packing  of  transit  plates 33 

XXVII. — Wet  emulsion  plates 33 

XXVIII.— Bath  wet  plates 33 

X  X IX. — General  precautions 34 

XXX. — The  chronograph    35 

XXXI. — Time  observations  and  chronometer  comparisons 35 

XXXIL — Exchange  of  time  with  other  parties 36 

X XXII I. — Latitude  and  longitude  of  station 36 

XXXIV. — The  equatorial  telescope 36 

X  XXV. — Occultations 37 

XXXVI. — General  instructions  respecting  observations  of  contacts 37 

X  X  XVII. — Methods  of  recording  contacts 44 

XXXVIII.— Diameter  of  Venus 45 

XX XIX. —  Data  required 46 

XL. — Records  of  observations  and  operations   49 

XLI. — Transmission  and  publication  of  observations 50 


NAVY  DEPARTMENT,  Washington,  August  u,  1882. 

The  following  instructions,  prepared  by  the  Commission  on  the  Transit  of  Venus, 
are  issued,  under  the  authority  of  this  Department,  for  the  guidance  and  government 
of  the  parties  charged  with  the  duty  of  making  the  observations  of  the  Transit.  Every 
person  engaged  in  the  scientific  operations  is  enjoined  to  study  them  carefully  and  to 
conform  to  them  strictly. 

WM.  E.  CHANDLER, 

Secretary  of  the  Navy. 


INSTRUCTIONS 

FOR 

OBSERVING  THE  TRANSIT  OF  VENUS. 


DECEMBER    6,    1883. 


I.— INTRODUCTORY  NOTE. 

The  following  instructions  were  prepared  primarily  for  the  use  of  the  parties  organ- 
ized by  the  U.  S.  Transit  of  Venus  Commission.  But  as  the  transit  will  be  visible  in 
this  country,  they  have  also  been  adapted  to  the  use  of  amateur  observers  who  desire 
to  be  made  acquainted  with  the  methods  by  which  they  may  make  observations  of 

value. 

II.— SELECTION  OF  STATION. 

In  choosing  ground  for  a  station,  it  is  most  important  to  obtain  good  foundations 
for  the  instruments.  Gravel  is  best,  but  a  sandy  soil  is  unobjectionable,  if  the  sand  is 
not  dry. 

A  nearly  level  spot,  measuring  at  least  sixty  feet  from  north  to  south,  is  necessary 
for  setting  up  the  transit  instrument  and  photoheliograph.  The  floor  of  the  photo- 
graphic house  must  be  eight  and  a  half  inches  lower  than  that  of  the  transit  house,  and 
it  will  be  advantageous  if  the  natural  slope  of  the  ground  gives  that  amount  of  fall ; 
but  if  it  does  not,  one  or  other  of  the  houses  must  be  elevated  to  obtain  the  proper 
difference  of  level  between  them.  For  the  sake  of  dryness,  the  sills  of  the  houses 
should  be  raised  three  or  four  inches  above  the  surface  of  the  soil.  In  many  situa- 
tions this  can  be  convenietly  done  by  driving  stakes  about  four  inches  square  into  the 
ground,  setting  the  houses  upon  them,  and  nailing  them  fast.  In  other  places  it  may 
be  necessary  to  use  small  piers  of  stone  or  brick. 

Shelter  from  the  prevailing  winds  is  to  be  sought  for;  but  great  care  must  be 
taken  that  the  sun  be  visible  from  the  point  occupied  by  the  photographic  objective, 
and  by  the  equatorial  telescope,  during  the  whole  time  of  the  transit,  and,  indeed,  a 
little  longer.  For  this  and  other  purposes,  the  observers  must  make  sure  of  the  exact 
local  times  of  all  the  contacts. 

After  the  site  for  the  station  is  selected,  a  survey  of  the  surrounding  region  must 
be  made  to  determine  the  location  of  the  station  relatively  to  neighboring  permanent 
objects.  If  a  large  scale  map  of  the  region  can  be  obtained,  the  position  of  the 
station  may  be  marked  upon  it;  but  if  no  such  map  is  available,  one  must  be  made. 

9 

2  INS 


10  INSTRUCTIONS  FOR  OBSERVING  THE 

A  carefully- written  description  of  the  position  of  the  station  must  also  accompany 
tin-  map.  The  object  of  these  records  is  to  provide  means  for  relocating  the  station 
within  one  or  two  hundred  yards  at  any  future  time. 

III.— LAYING  OFF  GROUND,  AND  SETTING  UP  INSTRUMENTS. 

The  approximate  position  of  the  meridian-line,  on  which  are  to  be  placed  the 
three  piers  for  supporting  the  transit,  the  photographic  objective,  and  the  plate-holder, 
being  determined  by  compass  or  otherwise,  the  next  two  operations,  which  may  be 
carried  on  together,  will  be  the  laying  down  of  an  exact  meridian-line  and  the  erection 
of  a  pier  for  the  transit. 

As  soon  as  the  position  of  the  transit  pier  is  decided,  a  point  should  be  selected 
about  seven  feet  east  or  west  of  its  center,  from  which  to  lay  off  a  meridian-line  by 
the  theodolite.  The  latter  instrument  being  placed  over  this  point,  a  stake  should  be 
driven  nearly  south  or  north  of  it,  and  at  a  distance  of  not  less  than  one  hundred 
feet;  and  then  the  azimuth  of  the  line  joining  the  instrument  and  the  stake  should  be 
found  from  observations  of  the  sun  or  stars  made  on  both  sides  of  the  meridian. 
From  this  line  of  known  azimuth  a  true  meridian-line  should  next  be  laid  down,  and 
that  can  be  done  either  with  the  theodolite,  or  by  setting  off  a  proper  offset  from  the 
stake.  Perhaps  the  latter  is  the  better  method. 

The  foundation  of  the  transit  pier  should  be  three  or  four  feet  below  the  surface 
of  the  ground,  and,  in  northern  stations,  below  the  reach  of  frost,  if  practicable.  The 
last  earth  at  the  bottom  of  the  excavation  should  be  carefully  removed  with  a  spade 
in  horizontal  slices,  so  as  to  produce  a  clean,  hard,  surface  for  the  footing  course  to  rest 
upon.  It  is  desirable  to  prevent  the  surface  earth  pressing  against  the  sides  of  the 
pier,  and  for  that  reason  a  vacant  space  one  or  two  feet  deep  should  be  left  around 
it — a  curb  being  used  to  keep  back  the  earth,  if  necessary.  The  sides  of  the  pier  must 
face  the  cardinal  points  accurately-  Above  ground,  it  should  measure  twenty-one 
inches  from  north  to  south,  and  twenty-five  inches  from  east  to  west;  below,  it  may 
be  from  three  to  three  and  a  half  feet  square.  The  cap-stone  must  be  twenty-four  by 
twenty-eight  inches,  and  three  inches  thick.  The  transit  house  must  be  built  around 
the  pier  after  the  latter  is  erected,  the  floor  of  the  house  1  icing  placed  twenty-nine 
inches  below  the  top  of  the  pier. 

In  the  northern  hemisphere  the  photographic  house  will  be  south,  and  in  the 
southern  hemisphere  north,  of  the  transit  house.  The  positions  and  depths  of  the  holes 
for  the  iron  piers  of  the  photoheliograph  can  be  readily  determined  by  means  of  the 
meridian  line  and  the  numbers  in  TAHLK  I.  The  holes  may  be  dug  at  any  time,  but 
the  piers  cannot  be  set  until  the  transit  instrument  has  been  brought  into  the  meridian 
and  its  errors  made  as  small  as  possible.  The  lower  ends  of  the  piers  have  thin  edges, 
and  to  prevent  them  settling  into  the  ground  they  should  be  placed  upon  large  Hat 
stones  in  the  bottoms  of  the  holes.  The  conditions  to  he  fulfilled  in  erecting  the  piers 
an-  as  follows: 

1.  The  flanged  ends  of  the  piers  must  he  uppermost. 

2.  The  larger  of  tile  two  piers  carries  the  photographic  objective,  and  goes  nearest 
the  transit  house.     Its  center  must  be  about  fourteen  feet  from  the  center  of  the  transit 
pier. 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  11 

3.  The  distance  between  the  centers  of  the  two  piers  must  be  as  stated  in  TABLE  I. 

4.  The  height  of  the  upper  surface  of  the  flange  of  each  pier,  or,  in  other  words, 
the  height  of  its  top,  must  be  as  stated  in  TABLE  I. 

5.  The  centers  of  the  two  piers  must  be  in  the  plane  of  the  true  meridian  pass- 
ing through  the  transit  instrument. 

6.  The  piers  must  stand  perfectly  plumb. 

7.  The  screw-holes  in  the  flanges  of  the  piers  must  be  so  placed  that  in  each  pier 
the  hole  nearest  the  transit  house  is  in  the  plane  of  the  meridian — that  is,  in  the  plane 
passing  through  the  centers  of  the  piers.     The  aperture  in  the  side  of  the  plate- 
holder  pier  must  also  be  in  the  meridian,  but  must  face  away  from  the  transit  house. 

The  simultaneous  fulfillment  of  so  many  conditions  is  troublesome.  A  convenient 
way  of  proceeding  will  be  to  tack  a  lath  across  the  slit  in  the  transit  house  at  a  point 
four  feet  above  the  floor,  and  from  it  to  stretch  a  cord,  perhaps  sixty  feet  long,  in  the 
meridian  of  the  transit  instrument  and  horizontally.  A  strip  of  wood  must  also  be 
jammed  into  the  top  of  each  iron  pier  in  such  a  way  that  a  five-penny  nail  can  be  driven 
into  it,  and  be  left  projecting  an  inch,  to  mark  the  center  of  the  pier.  Then,  by  stick- 
ing pins  through  the  line  at  the  points  beneath  which  the  centers  of  the  piers  must  be 
located,  the  latter  can  be  brought  very  approximately  into  position.  In  making  the 
final  adjustments,  an  engineer's  level  and  a  theodolite  will  be  required.  The  engi- 
neer's level  should  be  set  up  about  sixty  feet  from  the  transit  pier,  and  in  such  a  posi- 
tion that  the  latter  can  be  seen  through  the  door  of  the  transit  house.  The  theodolite 
must  be  mounted  behind  the  transit  house,  and  in  the  meridian  of,  but  a  little  higher 
than,  the  transit  instrument,  so  that  the  tops  of  the  iron  piers  can  be  seen  over  the 
latter.  By  moving  the  transit  instrument  in  altitude  only,  and  the  theodolite  in  both 
altitude  and  azimuth,  the  two  instruments  must  be  pointed  on  each  other,  and  the  im- 
age of  the  central  wires  of  the  transit  must  be  brought  into  coincidence  with  the  cent- 
ral wires  of  the  theodolite,  and  then  the  line  of  collimation  of  the  latter  will  describe 
a  plane  parallel  to  the  meridian  of  the  former.  This  suffices  for  our  purpose ;  but  if 
it  is  desired  to  bring  the  meridians  of  the  two  instruments  into  absolute  coincidence,  it 
can  be  done  by  shifting  the  theodolite  towards  the  east  or  west  until  the  images  of 
the  two  objectives  given  by  the  theodolite  eye-piece  are  seen  to  be  concentric.  For 
that  purpose  a  magnifier  is  necessaiy,  and  care  must  be  taken  that  the  coincidence  of 
wires  of  the  two  instruments  is  left  perfect.  Thus  adjusted,  the  theodolite  will  show 
with  great  precision  when  the  centers  of  the  piers  are  in  the  meridian  of  the  transit 
instrument ;  and  the  engineer's  level  will  show  equally  exactly  when  their  tops  are  at 
the  proper  height. 

Instead  of  using  a  theodolite  to  place  the  piers  in  the  meridian,  the  transit 
instrument  itself  may  be  employed,  if  its  objective  is  covered  by  a  cap  having  a  dia- 
metral slit  one-fifth  of  an  inch  wide.  This  slit  must  be  placed  truly  vertical,  and  then 
it  will  be  possible  to  obtain  somewhat  indistinct  vision  of  any  vertical  line  situated  in 
the  meridian  of  the  instrument  and  having  a  diameter  not  less  than  the  width  of  the 
slit.  If  the  centers  of  the  piers  are  marked  by  twenty-penny  nails,  instead,  of  five- 
penny  ones,  they  can  be  seen  through  the  transit.  To  guard  against  error,  after  a 
pier  has  been  apparently  brought  into  the  meridian,  the  cap  should  be  twisted  through 


12  INSTRUCTIONS  FOR  OBSERVING  THE 

half  a  revolution,  and  if  that  produces  any  change  in  the  position  of  the  image  in  the 
transit,  the  mean  of  the  two  positions  will  be  the  true  one,  and  the  center  of  the  pier 
must  be  adjusted  accordingly. 

The  pier  for  the  photographic  objective  should  first  be  brought  accurately  into 
position,  and  afterwards  the  same  thing  should  be  done  for  the  plate-holder  pier.  In 
adjusting  the  distance  between  the  two  piers  a  steel  tape-line  must  be  used,  not  a  linen 
one.  Nails  answer  well  as  wedges  for  making  small  changes  in  the  heights  of  the 
piers.  When  all  is  ready,  the  piers  must  be  finally  fixed  by  filling  the  holes  around 
them  with  masonry  laid  in  cement,  or  with  cement  concrete — earth  does  not  give  suffi- 
cient firmness;  and  in  doing  this  special  care  must  be  taken  that  they  are  not  accident- 
ally shifted  from  their  true  positions.  Their  interiors  should  also  be  filled  with  concrete 
to  the  level  of  the  ground. 

The  photographic  house  must  be  built  around  the  plate-holder  pier,  the  floor  of 
the  house  being  placed  exactly  three  feet  ten  and  three-quarter  inches  below  the  upper 
surface  of  the  flange  of  the  pier. 

The  cast-iron  plates  which  support  the  photographic  objective  and  plate-holder 
are  next  to  be  fixed  in  position.  The  coffin-shaped  plate  is  to  be  bolted  to  the  larger 
pier,  its  long  end  being  turned  toward  the  photographic  house,  and  at  least  one  washer 
being  placed  around  each  bolt,  between  the  plate  and  the  pier,  so  as  to  give  the  former 
a  solid  bearing.  The  coffin  plates  are  not  all  alike.  The  older  ones  have  pipes  at  the 
end,  to  receive  the  prongs  of  the  piece  which  supports  the  objective.  These  pipes  must 
be  turned  downward.  The  newer  ones  have  a  planed  seat,  upon  which  the  bottom  of 
the  support  for  the  .objective  is  bolted.  This  seat  must  face  upward.  When  the 
objective-carrier  is  first  mounted  upon  the  coffin  plate,  the  screw,  or  screws,  which 
hold  it  should  be  only  lightly  set  up;  but  afterwards,  when  all  has  been  found  right, 
they  must  be  firmly  turned  home,  and  the  pipes,  if  there  are  any,  may  be  filled  with 
plaster  or  cement.  The  round  cast-iron  disc  which  carries  the  plate-holder  is  to  be 
secured  to  the  pier  in  the  photographic  house  by  the  proper  supporting  and  binding 
screws,  care  being  taken  that  it  is  turned  in  the  right  direction. 

To  mount  the  plate-holder,  the  brass  cross  must  be  screwed  to  the  cast-iron  disc 
on  top  of  the  plate-holder  pier,  the  tube  of  the  cross  passing  down  through  the  hole 
in  the  disc.  The  vertical  axis  of  the  plate-holder  is  to  be  set  in  this  tube,  and  when 
the  plate-holder  is  rotated  its  ends  should  just  graze  the  raised  ends  of  the  cross  below 
it.  Milled-headed  screws,  passing  through  slots  in  pieces  at  the  ends  of  the  arms  of 
the  cross,  are  provided  for  fixing  tU,e  plate-holder  in  position,  with  the  reticule  plate 
at  right  angles  to  the  optical  axis  of  the  objective 

The  structure  for  carrying  the  iron  measuring-rod  and  the  tube  of  the  photohelio- 
graph  should  next  be  erected;  and  the  measuring-rod,  but  not  the  tube,  must  be  mounted. 
The  last  frame  of  the  structure  should  be  two  feet  distant  from  the  nearest  edge  of 
the  coffin-plate,  and  the  position  of  the  measuring-rod  must  be  parallel  to,  but  nine 
inches  above,  the  optical  axis  of  the  photographic  objective.  For  further  details,  con- 
sult section  XI. 


TEANSIT  OF  VENUS,  DECEMBEE  6,  1882. 


13 


IV.— MEMOBANDA  EESPECTING  THE  PHOTOHELIOGEAPH. 

For  convenience  of  reference  the  measurements  required  in  erecting  the  horizontal 
photoheliograph  are  here  recapitulated. 

The  houses  may  be  set  up  on  stakes  4"  square,  driven  firmly  into  the  ground. 
The  upper  surfaces  of  the  floors  should  be  7"  or  8"  above  the  surface  of  the  ground. 
The  distance  from  the  upper  surfaces  of  the  floors  to  the  bottoms  of  the  sills  is  about  3 £". 

Size  of  transit  pier:  below  ground,  3'  or  3'  6"  square;  above  ground,  2i"x2$". 
Capstone  for  ditto,  24"  x  28"  x  3".  Top  of  pier  above  floor,  29".  Eye-piece  of  transit 
above  floor,  3'  9^". 

The  iron  piers  for  the  photoheliograph  are  8'  o"  long.  The  larger  of  the  two 
carries  the  objective. 

Distance  from  center  of  transit  pier  to  center  of  pier  for  photographic  objective, 
14'  o". 

Photographic  house:  Floor  8 J"  lower  than  floor  of  transit  house.  Distance  from 
inner  side  of  wall  of  house  to  center  of  plate-holder  pier,  1 2".  The  top  of  this  pier 
is  9^"  above  top  of  transit  pier,  and  3'  lof"  above  floor  of  photographic  house.  Cen- 
ter of  plate-holder  above  floor,  4'  6". 

In  TABLE  I,  the  quantities  on  each  line  refer  to  a  single  photoheliograph.  The 
number  of  the  objective  is  given  in  column  A.  The  distance  between  the  back  sur- 
face of  the  objective  and  the  sensitive  surface  of  the  photographic  plate  is  given  in 
column  B;  it  being  assumed  that  the  thickness  of  the  reticule  plate  is  0.25  of  an  inch. 
The  distance  of  the  back  surface  of  the  objective  from  its  second  principal  point  is 
given  in  column  C.  The  distance  between  the  centers  of  the  piers  which  carry  the 
objective  and  plate-holder  is  given  in  column  D.  The  height  of  the  top  of  the  objective 
pier  above  the  top  of  the  transit  pier  is  given  in  column  E.  The  number  of  the  meas- 
uring-rod to  be  used  with  each  objective  is  given  in  column  F.  The  length  of  each 
measuring-rod,  at  62°  Fahrenheit,  is  given  in  column  Gr. 

TABLE  I. 


A. 

B. 

C. 

D. 

E. 

F. 

G. 

I 

Inches. 

462.51 

Inch. 
0.711 

Inches. 
477.6 

Inches. 
10.4 

VIII 

Inches. 

45I-49' 

2 

465.08 

.682 

. 

VII 

453-49S 

3 

462.27 

.674 

476.9 

9.4             II 

450-437 

4 

463.33 

.718 

478.8 

10.4           VI 

451-946 

5 

464.79 

•538 

479-9 

10.4 

III 

453.488 

6 

472.90 

.708 

488.5 

10.4 

I 

461.425 

7 

461.20 

.644 

476.4 

10.4 

v 

449-485 

8 

461.30 

0.708 

476.0 

9.4 

IV 

450.357 

The  expansion  of  the  measuring-rods  may  be  taken  as  0.0000070  of  their  length 
for  one  degree  Fahrenheit. 


14  INSTRUCTIONS  FOR  OBSERVING  Till' 

V.— ADJUSTMENTS  OF  THE  mOTOITELIOGRAPH. 

The  photoheliograph  must  fulfill  the  following  conditions: 

1.  The  sensitive  surface  of  the  photographic  plate  must  be  at  the  focus  of  the 
objective. 

2.  The  line  joining  the  optical  center  of  the  object-glass  and  the  cross-lines  in  the 
middle  of  the  reticule  plate  must  be  in  the  true  meridian,  within  a  fraction  of  a  minute 
of  arc. 

3.  The  same  line  must  be  horizontal,  within  the  same  limits. 

4.  The  optical  axis  of  the  objective  must  be  directed  toward  the  center  of  the 
reticule  plate. 

5.  The  reticule  plate  must  be  perpendicular  to  the  line  joining  its  center  and  the 
center  of  the  objective. 

6.  To  let  the  plumb  line  hang  freely,  the  sides  of  the  plate-holder  must  be  verti- 
cal; and  that  will  be  attained  by  making  its  top  level. 

These  adjustments  are  made  as  follows: 

1.  By  the  aid  of  the  measuring-rod,  set  the  coffin-plate  so  that  the  distance  between 
the  back  surface  of  the  objective  and  the  sensitive  surface  of  the  photographic  plate  is 
as  stated  in  column  B  of  TABLE  I.     To  guard  against  errors,  after  the  adjustments  i 
to  4  have  been  made,  remove  the  brass  plate-holder  and  use  the  wooden  one  to  take  a 
number  of  photographs  of  the  sun  at  different  distances  inside  and  outside  the  focus. 
The  points  at  which  the  small  spots  on  the  sun  begin  to  disappear  when  the  plate  is  too 
far  in,  and  again  when  it  is  too  far  out,  must  be  noted.     The  mean  of  the  two  positions 
is  the  true  focus.     If,  after  repeated  trials,  it  differs  more  than  one-quarter  of  an  inch 
from  the  point  found  by  measurement,  the  position  of  the  objective  must  be  changed 
accordingly. 

2.  When  the  photographic  objective  is  in   position,  point  the  telescope  of  the 
transit  instrument  at  it,  and  set  a  bull's-eye  lantern  behind  the  center  of  the  reticule- 
plate.     The  lines  upon  the  latter  will  then  be  visible  through  the  transit,  but  not  very 
distinctly,  because  the  photographic  focus  of  the  photoheliograph  differs  considerably 
from  its  visual  focus.    If  there  is  any  difficulty  in  seeing  and  identifying  the  intersection 
of  the  central  lines,  gum  a  small  triangular  bit  of  paper  upon  the  reticule-plate  with 
one  of  its  angles  at  the  point  in  question;  or  rule  a  little  cross  with  ink,  making  its 
lines  not  more  than  half  an  inch  long,  and  taking  care  that  they  coincide  exactly  with 
the  etched  lines  of  the  plate.     If  the  a/hmith  and  collimation  of  the  transit  are  quite 
right,  its  middle  vertical  wire  should  be  on  the  middle  vertical  line  of  the  reticule- 
plate.     If  it  is  not  found  so,  the  error  must  be  corrected  by  moving  the  brass  cross 
which  carries  the  plate-holder. 

3.  Point  the  transit  so  that  its  middle   horizontal  wire  accurately  coincides  with 
the  image  of  the  middle  horizontal  line  of  the  reticule-plate,  and  clamp  it  firmly  in  that 
position.     Then  .-H  ii]»  a  carefully  adjusted  engineer's  level  between  the  transit  house 
;unl  the  photographic  objective,  point  it  into  the  latter,  and  bring  its  horizontal  wire 
into  accurate  coincidence  with  the  image  of  the  middle  horizontal  line  of  the  reticule- 
plate.     Read  the  bubble  of  the  level,  and  if  it  is  within  six  or  eight  divisions  of  the 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  15 

middle  of  its  scale  the  height  of  the  plate-holder  is  probably  satisfactory.  To  make 
sure  of  this,  point  the  level  at  the  transit,  bring  its  horizontal  wire  into  exact  coinci- 
dence with  the  middle  horizontal  wire  of  the  latter,  and  again  read  its  bubble.  Half 
the  distance  traveled  by  the  bubble  between  the  two  readings  will  be  the  error  of  level 
of  the  center  of  the  reticule-plate. 

Probably  at  the  first  trial  the  bubble  will  run  all  the  way  to  one  end  or  the  other 
of  its  tube.  If,  at  each  pointing,  it  runs  to  the  end  nearest  the  photographic  house, 
the  reticule-plate  is  too  high;  but  if  to  the  end  nearest  the  transit  house,  the  reticule- 
plate  is  too  low.  In  either  case  the  error  must  be  corrected  by  changing  the  elevation 
of  the  plate-holder,  or  by  changing  the  elevation  of  the  coffin-plate,  or  by  both.  The 
height  of  the  plate-holder  is  controlled  by  the  adjusting-screws  of  the  iron  disk  sup- 
porting it,  and  the  altitude  of  the  coffin-plate  may  be  modified  by  increasing  or  djmin- 
ishing  the  number  of  washers  under  it. 

If  instead  of  an  engineer's  level  a  level  of  precision  is  employed,  its  telescope 
must  be  pointed  at  the  center  of  the  reticule-plate,  and  its  bubble  must  be  read, 
reversed,  and  read  again.  Let  the  difference  of  these  two  readings  be  A.  Then  the 
telescope  must  be  rotated  aboiit  its  optical  axis  through  half  a  revolution,  once  more 
pointed  at  the  center  of  the  reticule-plate,  and,  as  before,  the  bubble  must  be  read, 
reversed,  and  read  again.  Calling  the  difference  of  this  last  pair  of  readings  B,  the 
error  of  level  of  the  center  of^the  reticule-plate  will  be  \  (A+B).  The  object  in 
rotating  the  telescope  through  half  a  revolution  is  to  eliminate  its  collimation-error. 

4.  After  the  plate-holder  is  fixed  in  its  true  position,  adjust  the  objective  by  its 
three   supporting-screws  so  that  if  a  candle  in  the  photographic  house  is  held  in  the 
line  passing  through  the  centers  of  the  objective  and  reticule-plate,  its  three  reflections 
from  the  objective  will  also  lie  in  the  same  line.     Instead  of  a  candle,  it  is  sometimes 
convenient  to  use  a  reflector  consisting  of  a  card-board  disc  two  or  three  inches  in 
diameter,  witli  a  hole  one-quarter  of  an  inch  in  diameter  through  its  center.     In  that 
case,  the  three  images  of  the  hole  reflected  from  the  objective  must  lie  in  the  line  in 
question. 

5.  Adjust  the  reticule-plate  so  that  if  a  candle  in  the  photographic  house  is  held 
as  far  as  possible  from  it,  and  in  the  line  passing  through  its  center  and  the  center  of 
the  objective,  the  reflection  of  the  candle  from  the  reticule-plate  will  also  lie  in  the 
same  line.    Here,  again,  a  card-board  reflector  may  be  used  instead  of  a  candle.     The 
adjustment  is  made  in  altitude  by  the  screws  supporting  the  iron  disc  which  carries 
the  plate-holder,  and  in  azimuth  by  turning  the  latter  on  its  vertical  axis.     When  all 
is  right,  the  screws  confining  the  plate-holder  must  be  firmly  turned  home. 

If  this  adjustment  is  correctly  made  the  surfaces  of  the  reticule-plate  will  be 
vertical.  This  may  be  independently  tested  in  two  ways: 

(«).  Set  the  engineer's  level  outside  the  house,  at  the  height  of,  and  near  the  cen- 
tral line  joining  the  objective  and  plate-holder,  in  either  direction  from  the  latter,  and 
at  such  distance  from  it,  not  less  than  1 5  or  20  feet,  as  will  be  favorable  for  the  obser- 
vation. Point  the  level  at  the  plate-holder,  turn  the  latter  so  that  a  horizontal  line 
'upon  the  surface  of  the  reticule  shall  be  perpendicular  to  the  line  from  the  level,  and 
adjust  the  focus  of  the  level  so  that  its  objective  can  be  seen  by  reflection  from  the 


1C,  IN8BUGTION8  FOR  OBSERVING  THE 

reticule-plate.  If  the  plate  is  vertical,  and  the  level  properly  adjusted,  the  horizontal 
wire  of  the  level  will  bisect  the  reflected  image  of  the  objective.  The  latter  should 
return  to  its  position  when  the  plate-holder  is  turned  half-way  round  on  its  axis,  so 
that  the  reflection  takes  place  from  the  other  surface. 

(ft).  Adjust  the  base  of  the  plate-holder  so  that  the  bubble  of  a  level  set  upon 
its  top  shall  not  vary  greatly  in  position  when  the  holder  is  turned  on  its  axis. 

If  these  two  tests  cannot  be  both  satisfied  within  one  or  two  minutes  of  arc  by 
the  same  adjustment,  the  chief  should  endeavor  to  ascertain  what  is  wrong,  though  it 
may  not  be  advisable  for  him  to  try  to  correct  it. 

6.  The  verticality  of  the  sides  of  the  plate-holder  is  controlled  by  the  supporting 
screws  of  the  iron  disk  which  carries  it.  An  ordinary  carpenter's  level  suffices  to  show 
whei^  the  adjustment  is  correct. 

The  adjustments  from  i  to  6  are  necessarily  made  consecutively,  and  they  must 
be  gone  over  a  second  time  to  make  sure  that  the  later  ones  have  not  disturbed  those 
first  established. 

VI.— TIIE  HELIOSTAT. 

As  the  heliostat  will  presently  be  needed,  it  should  now  be  set  in  front  of  the 
objective,  upon  the  coffin-shaped  plate,  and  the  three  adjustments  which  it  requires 
should  be  made.  They  are  as  follows:  i.  Its  main^ixis  must  be  brought  into  the 
plane  of  the  meridian;  2.  Its  main  axis  must  be  set  at  that  inclination  which  will 
keep  the  sun's  image  most  nearly  at  a  constant  height  upon  the  reticule  plate;  3.  The 
driving-clock  must  be  arranged  to  rotate  the  main  axis  at  the  proper  speed.  Extreme 
accuracy  in  these  adjustments  is  superfluous,  because,  in  order  to  obtain  freedom  from 
vibration,  a  form  of  heliostat  has  been  adop'ted  which  can  throw  the  sun's  rays  only 
approximately  in  a  constant  direction.  A  convenient  way  of  proceeding,  and  one 
which  will  probably  give  as  good  results  as  any,  will  be  to  make  the  first  adjustment 
by  estimation,  using  a  ruler  laid  against  the  cube  of  the  main  axis  to  aid  the  judgment; 
and  then  to  effect  the  second  and  third  adjustments  by  trial. 

If  more  exactness  is  desired,  the  first  adjustment  may  be  made  by  setting  the 
mirror  at  right  angles  to  the  main  axis  of  the  heliostat,  and  then  setting  the  latter  so 
that  the  mirror  is  also  at  right  angles  to  the  axis  of  the  photographic  telescope.  The 
setting  of  the  mirror  is  accomplished  when  the  direction  of  a  ray  reflected  from  its  first 
surface  is  not  affected  by  rotating  the  main  axis;  but  in  applying  this  test  care  must 
be  taken  not  to  mistake  the  ray  reflected  from  the  second  surface  for  that  from  the 
first.  Then  the  main  axis  is  brought  into  the  plane  of  the  meridian,  and  made  hori- 
zontal, by  setting  the  heliostat  so  that  it  will  reflect  back  upon  itself  the  light  from  a 
candle  held  at  the  center  of  the  reticule-plate.  The  proper  inclination  for  the  main 
axis  may  be  computed,  and  the  axis  can  he  set  by  means  of  a  clinometer;  but  the 
rating  of  the  driving-clock  can  only  be  effected  by  trial. 

Whenever  tin-  a/imuth  and  level  of  the  center  of  the  reticule-plate  are  determined, 
the  heliostat  will   have   to   lie  removed  from   the   coffin   plate:   and   to  save  trouble  in  t 
returning  it  to  its  proper  place,  the  points  where  its  feet  rest  should  be  marked. 


TRANSIT  OP  VENUS,  DECEMBER  6,  1882.  17 

The  driving-clock  is  provided  with  three  pulleys,  whose  time  of  revolution  may 
be  varied  from  37.8  seconds  to  42.3  seconds  by  raising  or  lowering  the  pendulum  bob. 
The  screw  of  the  heliostat  carries  two  wheels,  either  of  which  can  be  connected  by  a 
leather  band  to  any  one  of  the  driving-clock  pulleys;  and  by  making  suitable  combi- 
nations, the  screw  can  be  driven  at  any  desired  speed  between  the  limits  58.8  seconds 
and  115.0  seconds  per  revolution.  This  suffices  for  all  localities.  Sometimes  the 
pendulum  of  the  driving-clock  takes  a  wabbling  motion,  moving  in  an  ellipse  instead 
of  a  circle.  When  this  happens  it  is  generally  occasioned  by  friction  at  the  point  of 
suspension,  and  a  little  oil  will  remedy  the  difficulty. 

VII.— THE  EXPOSING  SLIDE. 

The  frame  carrying  the  exposing  slide  must  be  screwed  to  the  inner  surface  of 
the  wall  of  the  photographic  house  in  such  a  position  that  the  line  joining  the  centers 
of  the  objective  and  reticule-plate  passes  through  the  center  of  the  opening  in  the 
frame.  Upon  each  end  of  that  surface  of  the  slide  which  is  nearest  the  objective 
a  target  is  painted,  and  whenever  the  slide  is  brought  into  contact  with  the  pieces 
which  limit  its  motion,  one  other  of  these  targets  covers  the  aperture  in  the  frame, 
arid  is  visible  to  a  person  standing  at  the  objective.  If  the  image  of  the  sun  given  by 
the  latter  is  then  centered  upon  the  target,  it  is  intended  that  it  shall  also  be  found 
centered  upon  the  reticule-plate  when  the  slide  is  moved  across  the  opening  in  the 
frame.  To  secure  this  result,  tfie  adjustments  of  the  photoheliograph  should  be  com- 
pleted before  the  frame  is  put  up,  and  special  pains  should  be  taken  to  fix  it  exactly 
in  its  right  position. 

By  means  of  the  six  milled-headed  screws  upon  the  slide,  the  brass  plates  can  be 
set  so  as  to  give  any  desired  width  of  slit;  but  in  doing  this  care  must  be  taken  to 
keep  the  center  of  the  slit  coincident  with  the  center  of  the  opening  in  the  slide, 
because  the  automatic  key  for  recording  the  instant  of  exposure  upon  the  chrono- 
graph is  arranged  to  break  when  the  centers  of  the  openings  in  the  slit  and  its  frame 
coincide  with  each  other.  It  is  sometimes  desirable  to  see  the  entire  image  of  the 
sun  upon  the  reticule-plate,  and  the  slit  is  arranged  to  open  wide  enough  for  that. 

VIII.— THE  TUBE. 

Experience  has  shown  that  for  a  photoheliograph  of  thirty-eight  and  a  half  feet 
focus  twelve  feet  of  tube  is  sufficient.  For  convenience  of  transportation,  that  fur- 
nished to  the  parties  is  slightly  conical,  and'  in  two-feet  lengths  packed  inside  each 
other.  In  mounting  the  tube,  four  points  require  attention,  namely:  i.  The  largest 
end  of  the  tube  should  pass  snugly  through  the  wall  of  the  photographic  house  and 
rest  against  the  back  of  the  exposing-slide  frame.  2.  The  centers  of  the  openings  in 
all  the  diaphragms  should  be  in  the  straight  line  joining  the  centers  of  the  objective 
and  reticule-plate.  3.  From  the  reticule  plate  nothing  but  black  surfaces  should 
be  visible.  The  presence  of  white  light  risks  fogging  the  photographic  plates,  and 
therefore  this  condition  is  imperative.  To  fulfill  it,  that  one  of  the  frames  carrying  the 
measuring-rod  which  is  nearest  the  objective  must  have  its  upper  part  boarded  over 
so  as  to  shut  out  extraneous  light  from  the  tube.  Both  surfaces  of  this  screen  should 
3  INS 


18  INSTRUCTIONS  FOR  OBSERVING  THE 

be  colored  dead  black,  and  a  bole  must  be  cut  through  it  just  large  enough  to  permit 
the  free  passage  of  rays  from  all  parts  of  the  objective.  4.  It  is  absolutely  necessary 
that  both  the  tube  and  the  measuring-rod  be  thoroughly  protected  from  the  sun's  rays. 
As  there  must  be  an  air-space  of  from  six  to  twelve  inches  around  the  tube,  the  root' 
or  awning  covering  it  should  have  a  depth  not  less  th;in  three  and  one-quarter  feet. 
Beyond  the  termination  of  the  tube,  the  covering  for  the  meairaring-rod  may  be  formed 
of  three  boards,  each  eight  inches  wide,  put  together  so  MS  to  form  a  kind  of  inverted 
trough. 

IX.— THE  PLATE-HOLDER  PLUMB-LINK. 

Both  the  chief  of  party  and  the  chief  photographer  must  give  special  attention 
to  the  plate-holder  plumb-line.  The  wire  employed  is  of  gilded  brass,  having  a  length 
of  about  three  feet  and  a  diameter  of  0.0025  °f  an  inch.  It  must  be  without  any 
bends  or  kinks,  must  pass  perfectly  freely  through  the  axis  of  the  plate-holder,  and 
must  be  loaded  with  one-fifth  of  its  breaking  weight  To  secure  steadiness,  the  weight 
must  hang  in  a  vessel  of  water  within  the  pier,  care  being  taken  that  the  vessel 
has  sufficient  size  and  is  so  placed  as  to  avoid  any  risk  of  the  weight  resting  against 
its  bottom  or  sides.  The  upper  extremity  of  the  plumb-line  is  wedged  into  a  brass 
piece  which  fits  into  a  socket  in  the  top  of  the  plate-holder,  and  is  provided  with  an 
arm  for  rotating  the  wire  through  half  a  revolution,  so  as  to  eliminate  the  effect  of  any 
undetected  kinks  or  bends.  Aside  from  this  motion  of  rotation,  the  plumb-line  should 
be  disturbed  as  little  as  possible. 

The  chief  of  party  must  satisfy  himself  from  time  to  time  that  the  plumb-line 
does  not  come  into  contact  with  anything  in  its  passage  through  the  opening  in  the 
axis  of  the  plate-holder.  <  >nc  way  of  testing  this  will  be  to  let  the  bob  swing  through 
a  minute  arc,  and  see  that  the  swinging  motion  of  the  suspending  wire  across  the  face 
of  the  reticule  plate,  as  viewed  with  a  magnifying  glass,  is  perfectly  regular. 

X.— BATTERIES  AND  ELECTRICAL  CONNECTIONS. 

The  batteries  furnished  to  the  parties  are  of  the  Daniell's  gravity  form.  To  set 
up  a  cell,  unfold  the  copper  element;  place  it,  together  with  about  two  pounds  of 
coarsely-powdered  copper  sulphate,  in  the  bottom  of  the  glass  jar;  and  lead  the  insu- 
lated copper  terminal  out  over  the  top  of  the  jar.  If  the  cell  is  desired  for  immediate 
use,  fill  it  to  within  one  and  a  half  inches  of  the  top  with  water  in  which  a  little  zinc 
sulphate  has  been  dissolved;  and  suspend  the  zinc  element  in  this  solution  by  hooking 
it  to  the  top  of  the  jar.  If  the  cell  is  not  required  immediately,  it  is  better  to  fill  it 
with  pure  water,  and  then  to  connect  the  copper  terminal  with  the  /.inc.  In  a  few 
hours  it  will  be  in  good  working  order.  When  using  these  cells  care  must  be  taken 
that  the  zinc  sulphate  solution  does  not  become  too  strong,  that  it  covers  the  zinc  ele- 
ment, and  that  there  are  always  somouopper  sulphate  crystals  in  the  bottom  of  the  jar. 

No  stronger  cm-rent  than  that  from  a  single  Daniell's  cell  should  ever  be  passed 
through  a  break-circuit  chronometer;  and  as  that  will  not  usuallv  suffice  for  working 
a  chronograph,  the  chronometer  must  be  joined  up  in  circuit  with  one  cell  of  battery, 
a  single-point  switch,  and  a  repeater  of  about  six  ohms  resistance.  The  break-circuit 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  19 

points  in  the  chronometer  are  very  delicate,  and  to  diminish  the  spark  at  them  the  lid 
of  the  chronometer  box  contains  a  condenser  which  should  be  included  in  the  circuit. 
The  chronograph  must  be  joined  up  in  a  second  circuit,  passing  through  the  points  of 
the  repeater,  and  including  the  observing  key  at  the  transit  instrument,  the  automatic 
key  of  the  exposing  slide  in  the  photographic  house,  and  such  a  number  of  battery 
cells  as  may  be  necessary.  Thus  the  chronometer  circuit  will  control  the  chronograph 
circuit,  and  by  opening  the  switch  in  the  former,  both  circuits  will  be  opened.  It  will 
be  well  to  arrange  the  wires  in  such  a  way  that  the  automatic  key  in  the  photographic 
house  can  be  cut  out  when  not  in  use.  As  a  rule,  only  break-circuit  signals  will  be 
used,  but  the  observing  keys  furnished  to  the  parties  have  a  screw  at  the  back,  by 
shifting  which  they  can  be  converted  into  make-circuit  keys  if  desired. 

XL— THE  MEASURING  ROD. 

The  most  convenient  way  of  supporting  the  measuring-rod  will  be  to  pass  it 
through  holes  one  and  one-quarter  inches  in  diameter  in  boards  nailed  across  the  tops 
of  A -shaped  frames  placed  not  more  than  six  feet  apart.  Whatever  mode  of  support 
is  adopted,  special  care  must  be  taken  to  see  that  the  rod  lies  perfectly  straight,  and 
that  it  is  parallel  to  the  optical  axis  of  the  photographic  telescope. 

The  rod  is  of  wrought-iron  gas-pipe,  eight-tenths  of  an  inch  diameter,  in  sections 
five  feet  long,  with  Arabic  numbers  at  the  joints  to  show  how  they  go  together.  The 
Roman  numbers  in  the  middle  of  each  section  are  the  number  of  the  rod.  The  ends 
of  each  section  must  be  carefully  cleaned  with  an  oily  rag  before  they  are  united, 
and  in  screwing  them  together  their  shoulders  must  be  made  to  meet  snugly.  Marks 
at  each  joint  show  approximately  the  point  of  stopping.  If  these  marks  fail  to  come 
together,  there  is  dirt  in  the  joint ;  but  if  they  pass  each  other  slightly,  it  only  indicates 
that  the  screw  is  worn,  which  is  of  no  consequence,  because  the  length  of  the  rod 
depends  upon  the  condition  of  the  shoulders  and  not  upon  that^of  the  screws.  To 
avoid  straining  the  rod  in  putting  it  together,  the  first  two  sections  should  be  united 
and  passed  on  to  the  supporting  frames  nearest  the  objective;  then  the  third  section 
should  be  added,  and  the  rod  should  be  pushed  nearer  the  photographic  house;  and 
so  on  till  it  is  all  together,  and  in  place.  The  end  inside  the  photographic  house 
should  be  at  the  same  distance  from  the  front  surface  of  the  reticule  plate  that  the 
end  outside  is  from  the  back  surface  of  the  objective.  After  being  mounted,  the  rod 
should  not  again  be  disturbed  till  the  party  is  about  to  leave  the  station.  Its  outer 
extremity  should  be  slightly  greased  to  prevent  it  from  rusting,  and  must  be  protected 
from'the  weather  by  the  tin  cover  furnished  for  that  purpose. 

The  measuring-rod  is  used  in  connection  with  the  jaw-micrometer  for  determining 
the  interval  between  the  back  surface  of  the  objective  and  the  front  surface  of  the 
reticule  plate.  To  do  this,  three  thermometers  must  first  b.e  placed  'upon*  the  rod  to 
ascertain  its  temperature,  one;  being  at  its  center  and  one  near  each  end.  Then  a 
plumb-line,  consisting  of  a  brass  wire  0.0032  of  an  inch  in  diameter,  must  be  hung 
over  the  outer  end  of  the  rod,  the  bob  of  the  line  hanging  in  ;i  vessel  of  water,  and 
being  protected  from  wind  by  complete  immersion  in  the  fluid.  Care  must  be  taken 
that  the  wire  bends  sharply  over  the  end  of  the  rod,  and  is  in  actual  contact  with  its 


20  INSTRUCTIONS  FOR  OBSERVING  THE 

front  surface.  When  the  plumb-line  has  come  to  rest,  press  the  arms  of  the  jaw- 
micrometer  against  the  margin  of  the  object-glass,  taking  great  care  to  hold  the 
micrometer  horizontally,  and  by  the  two  rack  motions  bring  the  end  of  the  central 
arm  gently  into  contact  with  the  back  surface  of  the  objective,  and  the  jaws  into  such 
a  position  that  the  plumb-line  is  between  them  and  in  the  line  joining  the  centers  of  the 
pin-holes.  In  adjusting  the  position  of  the  pin-holes  relatively  to  the  plumb-line,  it 
will  be  advantageous  to  use  a  magnifier  of  low  power.  When  all  the  adjustments  arc 
correct,  read  and  record  the  vernier  of  the  micrometer.  After  making  several  such 
measurements  at  the  objective,  pass  to  the  plate-holder  and  make  similar  measure- 
ments from  the  front  surface  of  the  reticule  plate.  Each  measure  should  be  repeated 
a  number  of  times  by  different  observers,  and  each  separate  result  should  be  recorded 
with  the  name  of  the  observer,  the  temperature  of  the  rod,  and  any  other  necessary 
particulars. 

Should  the  rod  be  too  long,  file  a  vertical  notch  on  each  side  near  one  end,  loop 
the  plumb-line,  and  let  it  hang  in  these  notches. 

The  distance  from  the  front  surface  of  the  reticule  plate  to  the  position  of  the 
collodion  film  must  also  be  measured  as  accurately  as  possible. 

XII.— INSTRUMENTAL  ERRORS. 

When  the  adjustments  of  the  photoheliograph  designated  1,4,  5,  and  6,  in  sec- 
tion V,  have  once  been  made,  it  is  expected  they  will  remain  sufficiently  exact,  unless 
purposely  disturbed.  If,  by  any  chance,  either  4  or  5  is  found  wrong,  it  must  be  cor- 
rected. The  errors  of  i,  2,  and  3  must  be  determined  at  least  twice  a  week,  but 
unless  they  become  large  they  need  not  be  corrected. 

To  find  the  error  of  the  first  adjustment,  the  distance  from  the  back  surface  of 
the  objective  to  the  front  surface  of  the  reticule  plate,  nmsr  be  measured  with  the 
greatest  care  at  times  when  the  temperature  is  not  varying  rapidly.  Directions  tin- 
doing  this  are  given  in  the  preceding  section. 

The  error  of  the  second  adjustment  must  be  found  as  follows :  K very  evening 
before  beginning  work  with  the  transit  instrument,  place  its  eye-piece  at  the  same  side 
of  the  stand  as  the  azimuths!  adjusting  screws,  and  by  means  of  the  latter,  bring  the 
middle  vertical  wire  of  the  transit  into  exact  coincidence  with  the  image  of  the  middle 
vertical  line  of  the  reticule  plate.  After  this,  the  a/imuthal  adjustment  must  remain 
undisturbed  during  the  night's  work.  Then  make  a  set  of  time  observations;  that  is, 
observe  two  azimuth  stars  above  the  pole,  two  below  the  pole,  and  six  or  eight  time 
stars,  one  half  the  observations  of  each  class  being  made  with  clamp  east  and  the 
other  half  with  clamp  west.  At  the  close  of  the  night's  work  again  bring  the  eye- 
piece to  the  same  side  of  the  stand  with  the  a/imuthal  screws,  and  examine  the  posi- 
tion of  the  middle  wire  relatively  to  the  image  of  the  middle  vertical  line  of  the 
reticule  plate.  If  there  is*  any  deviation,  its  amount  must  be  estimated  and  recorded, 
but  the  a/hmith  of  the  transit  must  not  be  disturbed. 

l'|)""  reducing  the  time  observations  the  ax.imuth  and  collimation  constants  of 
the  tran-it  will  become  known.  Let  them  be  <i  and  r,  and  let  the  azimuth  of  the  cen- 
ter of  the  reticule  plate,  counted  fn.in  the  meridian  toward  the  left,  be  A.  Then 

A  =  -«  +  r) 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  21 

There  will  be  two  values  of  c,  one  for  clamp  east  and  the  other  for  clamp  west. 
The  one  to  be  used  is  that  corresponding  to  the  position  of  the  instrument  when  its 
middle  wire  was  set  upon  the  center  of  the  reticule  plate.  If  A  exceeds  one  second 
of  time  for  several  nights,  the  plate-holder  must  be  adjusted  by  moving  it  one-thirtieth 
of  an  inch  for  each  second  of  error. 

The  method  of  determining  the  error  of  the  third  adjustment  has  been  already 
described  in  section  V. 

XIII.— FITTINGS  OF  THE  PHOTOGRAPHIC  HOUSE. 

The  photographers  will  examine  the  photographic  house  as  soon  as  it  is  erected, 
and  see  that  all  white  light  is  perfectly  excluded;  A  single  crack  in  the  wall,  or  even 
an  unprotected  keyhole  may  cause  irreparable  mischief.  Such  openings  are  readily 
detected  from  the  inside,  and  when  found  may  be  stopped  up  or  covered  with  yellow 
paper  or  other  suitable  material.  In  full  daylight  the  orange-glass  window  will  ad- 
mit too  much  light,  and  must  be  covered  with  orange  envelope  paper. 

The  emulsion  chest,  drying  box,  and  plate  boxes  are  also  to  be  examined,  and 
any  cracks  or  other  openings  closed  up.  The  drying  box  may  be  screwed  to  battens 
fixed  against  the  side  of  the  house,  or,  if  more  convenient,  it  may  be  mounted  on  legs. 
To  facilitate  cleaning,  the  top  of  the  box  lifts  off,  and  the  bottom  may  be  removed  by 
taking  out  a  couple  of  screws. 

Close  to  the  left-hand  side  of  the  pier  carrying  the  plate-holder  a  stand  large 
enough  to  support  a  chronometer  and  the  blank  form  for  recording  the  exposure  of 
plates  will  be  required.  A  shelf  long  enough  to  hold  two  plate-boxes,  side  by  side, 
will  be  fixed  to  the  wall  of  the  dark  room  close  to  the  right-hand  side  of  the  pier,  and 
at  a  convenient  height  for  taking  plates  from,  and  returning  them  to,  the  boxes. 

A  screen  should  be  interposed  between  the  plate-holder  and  the  boxes  on  the  shelf, 
so  that  if  the  slide  were  moved  while  the  boxes  were  open  (which  ought  never  to  hap- 
pen) no  light  could  fall  on  the  plates.  That  part  of  the  opposite  wall  which  receives 
the  sunlight  passing  through  the  plate-holder  should  be  blackened,  or  covered  with 
dark-colored  cloth,  so  as  not  to  reflect  light  about  the  room. 

If  external  objects  illuminated  by  direct  sunlight  are  visible  from  any  part  of  the 
plate-holder  in  any  position  of  the  slide,  foggy  plates  and  blurred  images  may  be  ex- 
pected, and  under  circumstances  requiring  long  exposures  any  light-colored  surface, 
exposed  to  radiation  from  the  sky  may  produce  similar  effects. 

An  abundant  supply  of  good  water  is  indispensable.  For  developing  dry  plates 
almost  any  well,  spring,  or  river  water  may  be  used,  even  if  it  contains  rather  large 
quantities  of  some  salts  and  is  not  entirely  free  from  organic  matter.  If  it  should  be 
necessary  to  prepare  silver  baths,  water  must  be  distilled  if  it  cannot  be  otherwise 
obtained  of  sufficient  purity. 

XIV.— CARE  OF  THE  SENSITIVE  EMULSION. 

The  sensitive  collodio-bromiile  emulsion  is  contained  in  bottles  of  orange  glass, 
only  partially  filled  to  facilitate  efficient  shaking.  After  standing  undisturbed  for  a 
long  time  the  silver  bromide  partly  subsides  to  the  bottom  of  the  bottle,  but  it  may  be 
perfectly  re-emulsified  by  agitation.  By  inverting  the  bottle  and  looking  through  its 


22  INSTRUCTIONS  FOR  OBSERVING  THE 

bottom  toward  the  dark-room  window,  a  more  or  less  abundant  deposit  will  be  seen, 
Mini  the  shaking  should  be  persevered  in  until  this  is  entirely  broken  up  and  washed 
away.  When  this  has  once  been  thoroughly  effected  the  bromide  will  be  easily  kept 
in  suspension  by  a  little  shaking  at  intervals  of  a  day  or  two.  Collodion  vials  con- 
taining emulsion  should  always  be  well  shaken  and  then  allowed  to  stand  a  few  minutes 

before  coating  plates.     As  issued  it  contains  a  certain  amount  of  coarse  sediment 

'  * 

which  must  be  removed  by  filtration. 

The  emulsion  must  not  be  exposed  to  white  light,  for  although  the  bottles  con- 
taining it  are  of  colored  glass  they  cannot  be  absolutely  relied  on  to  protect  it  from 
injury.  It  is  true  that  some  collodion  emulsions  may  be  submitted  for  a  slfort  time 
to  the  action  of  weak  daylight  without  material  deterioration,  but  in  other  cases  only 
foggy  images  can  be  obtained  after  such  treatment.  Additional  caution  is  requisite 
with  emulsion  that  has  been  transferred  to  collodion  vials  of  colorless  glass.  The 
greatest  care  must  also  be  taken  not  to  contaminate  the  emulsion  by  flowing  it  upon 
glass  that  is  not  perfectly  clean,  or  by  putting  it  into  bottles  that  have  been  exposed 
to  light  with  traces  of  emulsion  adhering  to  them. 

XV.- SELECTING  AND  MARKING  GLASS. 

The  photographic  operations  will  be  begun  by  examining  the  stock  of  glass  and 
rejecting  all  plates  that  are  broken  or  cracked,  not  sufficiently  flat,  with  a  rough  or 
uneven  surface,  too  large  to  enter  the  grooves  of  the  plate-boxes  easily,  or  having  any 
corner  so  short  that  it  will  not  rest  securely  on  the  pins  in  the  plate-holder  of  the 
photoheliograph.  A  sufficient  quantity  of  the  best  glass,  not  less  than  204  plates,  is 
next  to  be  selected  for  the  transit  plates,  and  after  choosing  the  best  surface  for  the 
front,  or  film  side,  each  plate  is  to  be  marked  in  one  corner  of  the  back  with  a  number, 
beginning  with  nnity  and  proceeding  consecutively  upward.  This  is  to  be  done 
neatlv  and  legibly  with  a  writing  diamond,  and  to  facilitate*  reference  to  the  finished 
photographs  when  stored  in  plate-boxes  the  numerals  should  be  so  placed  as  to  be 
upright  in  the  right-hand  upper  corner  of  the  plate. 

xvi.— CLEANING  AND  ALIUTMENIZING  GLASS. 

Remove  the  sharp  edges  with  a  whetstone,  and  clean  the  glass  from  any  gross 
impurity  that  mav  lie  adhering  to  it.  Old  films  are  best  got  rid  of  by  soaking  for  a 
few  hours  in  a  moderately  strong  solution  of  concentrated  lye,  alter  which  they  can 
generally  be  washed  away  without  much  labor;  but  the  surface  of  the  glass  may  be 
injured  by  leaving  it  too  long  in  such  a  solution,  and  especially  by  allowing  the  latter 
to  dry  upon  the  plate. 

The   chromic    solution    recommended    bv    Mr.  M.('.    Lea   is   best   adapted    to   the 
circumstances  of  the  transit  of  Venus   photographers,  as  it  rapidly  oxidi/.es  organic^ 
impurities  and  gives  off  no  vapor  injurious  to  instruments. 

Solltlioit  fur   Cli/llilil/l   (i/tlsN. 

liichromate  of  potash  .)  ounces. 

Sulphuric  acid  6  fluid  ounces. 

Water     -  50  fluid  ounces. 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  23 

Put  the  bichromate  of  potash  into  a  two-quart  bottle  and  pour  in  the  water. 
Then  add  (out  of  doors)  the  sulphuric  acid,  a  little  at  a  time,  shaking  the  bottle  Veil 
and  allowing  it  to  stand  a  few  minutes  after  each  addition.  It  Avill  become  very  hot 
and  give  off  a  little  corrosive  vapor,  which,  however,  soon  disappears.  If  too  much 
acid  is  added  at  once  the  bottle  will  probably  be  broken  by  the  heat.  When  the 
bichromate  of  potash  is  all  dissolved,  and  the  solution  has  cooled,  it  is  ready  for  use. 
Pour  it  into  a  large  rubber  pan,  and  immerse,  one  at  a  time,  as  many  plates  as  the 
solution  will  cover.  If  any  bubbles  are  allowed  to  remain  between  the  plates  they  will 
prevent  the  solution  from  acting  on  the  surfaces  in  contact  with  them.  The  glass 
should  remain  in  the  chromic  solution  through  one  night  at  least,  and  a  still  longer 
time  may  be  desirable,  but  it  ought  not  to  exceed  a  week,  or  thereabout. 

Remove  the  plates  from  the  cleaning  solution  and  put  them  into  a  pan  or  bucket 
of  clean  water.  The  solution  may  be  repeatedly  used ;  it  will  not  injure  the  skin 
beyond  causing  a  slight  stain  arid  a  rather  disagreeable  odor,  but  it  is  very  destructive 
to  clothing.  Renew  the  water  in  the  bucket,  and  separate  the  plates  from  each  other 
until  the  yellow  fluid  adhering  to  them  is  entirely  washed  away;  then  refill  the  bucket 
so  as  to  leave  them  entirely  covered  with  clean  water. 

The  plates  are  now  to  be  taken  one  at  a  time  from  the  bucket  and  rubbed 
with  the  fingers,  or  with  a  clean  cloth,  on  both  sides  and  around  all  the  edges  while 
the  water  from  the  tap  flows  on  them.  After  a  final  rinse  they  will  be  ready  for  albu- 
ineriizing. 

Albumen  Solution 

White  of  egg  i  fluid  ounce. 

Water  16  fluid  ounces. 

Strong  ammonia  15  minims. 

Put  the  white  of  egg  into  a  clean  bottle  of  convenient  size  together  with  a  few 
pieces  of  broken  glass,  shake  it  vigorously  until  the  albumen  is  thoroughly  "  beaten," 
and  allow  it  to  stand  undisturbed  for  several  hours.  Then  add  the  water  and  ammonia, 
and  shake  just  enough  to  mix  the  contents  of  the  bottle,  after  which  it  should  again 
stand  for  some  time  before  being  filtered  for  use.  If  tolerably  fresh  eggs  cannot  be 
obtained,  100  grains  of  dried  albumen  may  be  taken  as  the  equivalent  of  i  fluid  ounce 
of  white  of  egg,  but  the  latter  is  much  to  be  preferred. 

Filter  the  albumen  solution  into  an  8-ounce  graduate  until  it  is  two-thirds  filled, 
keeping  the  neck  of  the  funnel  under  the  surface  of  the  filtered  albumen  to  avoid 
bubbles.  The  few  bubbles  that  unavoidably  form  may  be  removed  with  a  wisp  of 
clean  paper. 

When  a  plate  has  been  washed  arid  rinsed  as  already  described,  observe  by  the 
mark  on  the  back  which  is  the  front  side,  and  flow  it  with  the  albumen  solution. 
After  draining  the  plate  for  a  few  seconds,  flow  it  a  second  time  precisely  as  before 
and  set  it  on  a  rack  to  dry,  always  keeping  the  same  corner  downward,  and  never 
touching  the  front  surface  or  the  uppermost  edges.  The  plates  will  be  apparently  dry 
in  the  course  of  an  hour  or  two  after  albumenizing,  but  the  albumen  should  be  allowed 
to  become  thoroughly  desiccated  arid  hardened  by  keeping  in  a  dry  place,  properly 


24  INSTRUCTIONS  FOB  OBSERVING  THE 

protected  against  dust,  for  several  days  if  possible.  If  coated  with  emulsion  too  soon 
aftel-  albumeniy.ing,  the  films  will  be  more  liable  to  blister  and  rise  from  the  glass  dur- 
ing development,  ;i  misfortune  to  be  avoided  by  the  exercise  of  every  possible  precau- 
tion. The  albumeni/ed  plates  are  finally  to  be  examined,  and  any  imperfect  ones  set 
aside  to  be  cleaned  again. 

As  the  permanent  marks  on  the  plates  will  scarcely  be  visible  in  the  dark  room, 
write  the  number  of  each  one  with  a  blue  pencil,  or  with  ink,  in  large  and  plain  figures 
on  a  gummed  label,  and  attach  it  to  the  back  of  the  corresponding  plate  in  such  posi- 
tion that  when  the  latter  is  put  into  a  plate-box  the  permanent  mark  will  be  seen  in 
the  upper  right-hand  corner,  and  the  number  on  the  label  in  the  upper  left-hand  corner 
of  the  plate.  Re-examine  them  carefully  to  insure  that  the  two  numbers  on  each  plate 
are  identical,  and,  if  found  so,  store  them  in  plate-boxes  for  use  as  required. 

XVII.— COATING  PLATES. 

As  the  plates  are  necessarilv  exposed  for  a  considerable  time  to  whatever  light 
may  be  in  the  dark  room  while  they  are  being  coated,  it  is  necessary  to  proceed  with 
caution.  But  by  operating  near  the  window  while  the  emulsion  vials,  and  the  rack  hold- 
ing the  plates  Already  coated,  are  protected  from  direct  light  by  interposing  screens, 
it  is  possible,  to  work  with  both  ease  and  safety.  The  outer  door  must,  of  course,  be 
locked.  In  very  warm  weather  it  is  more  comfortable  to  coat  plates  at  night  with  the 
door  and  window  open  it'  there  are  no  gas  lamps  or  other  dangerous  lights  outside. 
It  will  be  safe  to  use  naked  candles  if  they  are  so  screened  that,  no  direct  light  from 
them  or  from  anv  nearly  white  object  strongly  illuminated  by  them  can  fall  on  the 
plates  or  the  emulsion.  Light  from  gas  or  coal-oil  lamps  is  more  actinic  than  that  of 
candles,  and  must  be  used  with  caution  if  at  all.  Orange-glass  lanterns  ought  to  be 
(juite  safe  if  candles  are  used  in  them,  but  it  must  not  be  taken  for  granted  that  t!«ev 
are  so. 

Set  one  of  the  racks  from  the  drying  box  where  no  direct  light  from  the  window 
can  fall  on  it,  and  have  at  hand  a  wide  camel's-hair  brush,  and  a  bottle  containing  a 
mixture  of  two  volumes  of  ether  and  one  volume  of  alcohol  for  thinning  the  emul- 
sion. Begin  by  thoroughly  shaking  the  bottle  of  emulsion  it  is  proposed  to  use  ;  then 
insert  a  tuft  of  filtering  cotton  in  the  throat  of  a  perfectly  clean  emulsion  filter  and 
moisten  it  with  a  little  of  the  thinning  mixture.  Set  the  filter  in  a  twelve-ounce  col- 
lodion vial,  pour  in  emulsion  from  the  bottle  until  it  is  nearly  full,  put  on  the  cover, 
and  set  an  orange  bell-glass  over  the  filter  and  vial  to  diminish  evaporation  and  pre- 
vent the  access  of  light.  If  the  tuft  of  cotton  is  too  large  and  too  closelv  packed, 
filtration  will  be  slow  and  tedious;  if  not  properly  inserted  it  will  permit  the  passage 
n|' small  particles  of  sediment,  itc.  The  emulsion  should  pass  through  in  a  rapid  suc- 
cession of  large  drops,  about  one  per  second.  It  may  run  in  a  thin  stream  at  first,  as 
its  pMMge  will  be  slower  \\hen  the  cotton  is  fullv  saturated  with  emulsion.  Replen- 
ish the  filter  from  the  emulsion  bottle  before  it  becomes  empty,  and  transfer  it  to  an- 
other collodion  vial  when  the  first  one  is  two-thirds  full.  While  plates  are  being  coated 
from  one  vial  the  other  will  be  filling,  and  so  on,  using  them  alternately. 

Take  a  plate  from  the  plate-box,  and,  holding  it  near  the  window,  see  if  the 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  25 

albumenized  surface  is  free  from  fibers  and  particles  of  dust.  Such  substances  may  be 
carefully  removed  with  the  brush,  but  actual  brushing  will  leave  marks  on  the  albumen. 
Holding  the  plate  by  the  corner  that  was  marked  with  the  writing-diamond,  coat  it 
with  emulsion,  precisely  as  if  it  were  collodion,  and,  as  soon  as  the  film  is  set,  put  the 
plate  in  the  rack.  As  emulsion  is,  under  ordinary  circumstances,  rather  less  fluid  than 
collodion  it  is  possible  that  operators  who  are  accustomed  to  use  thin  collodion,  and 
coat  their  plates  very  deliberately,  may  experience  some  difficulty  in"  obtaining  a 
uniform  film.  Pour  on  a  rather  liberal  dose  of  emulsion  and  cover  the  plate  quickly; 
then  incline  the  plate  but  little,  so  as  to  pour  off  very  slowly,  and  impart  to  it  whatever 
motion  may  be  necessary  to  prevent  the  formation  of  lines.  The  precautions  usual 
with  careful  operators  to  prevent  particles  of  dried  emulsion  on  the  lip  of  the  vial  from 
falling  upon  the  plate  must,  of  course,  be  observed.  It  is  sometimes  recommended  to 
pour  from  one  vial  and  to  drain  the  plate  into  another,  so  that  emulsion  once  poured 
out  may  be  filtered  before  it  is  used  again;  but  this  mode  of  operating  is  inconvenient, 
and  will  not  be  necessary  unless  the  atmosphere  of  the  dark  room  is  charged  with 
dust.  The  corner  by  which  the  plate  was  held  while  coating  with  emulsion  is,  of 
course,  left  uncovered;  by  looking  toward  the  window  through  this  clear  space  the 
permanently  marked  number  of  the  plate  may  be  read  in  the  dark  room,  though  with 
some  difficulty.  When  the  emulsion  becomes  too  thick  to  flow  well  (or  sooner  if 
particles  of  foreign  matter  are  seen  in  it)  add  as  much  of  the  mixture  of  ether  and 
alcohol  as  may  be  required  to  bring  it  to  the  proper  consistency,  pour  the  whole  into 
the  emulsion  bottle,  and  shake  it  until  well  mixed.  Then  transfer  the  filter  to  the 
empty  vial,  and  begin  using  the  full  one.  The  filter  must  not  be  allowed  to  become 
empty.  It'  the  vial  containing  it  is  getting  too  nearly  full  it  may  be  set  in  the  emul- 
sion bottle.  Do  not  dilute  the  emulsion  unnecessarily  by  using  the  thinning  mixture 
too 'freely. 

When  the  rack  is  full  of  plates  remove  it  to  the  highest  unoccupied  position 
in  the  drying-box,  and  avoid  all  further  risk  of  accident  to  them  by  closing  and  bolt- 
ing the  doors.  The  box  will  contain  \~2()  plates,  but  it  is  better  to  make  a  smaller 
number  at  one  operation,  filling  only  every  second  or  third  groove  of  each  rack. 
When  a  sufficient  number  of  plates  are  prepared  return  the  bottle  and  vials  to  the 
emulsion  chest,  and  wash  the  filter  perfectly  clean  before  the  adhering  emulsion 
becomes  dry  and  hard. 

Plates  freely  exposed  to  the  air  of  the  dark  room  would  be  dry  in  an  hour  or 
thereabout;  in  the  drying-box  a  longer  time  will  be  required,  and  several  hours  at 
least  should  be  allowed.  At  night  the- top  and  bottom  of  the  box  may  be  taken  off- 
When  quite  dry  the  plates  are  to  be  put  in  plate-boxes  that  are  clean  and  free  from 
dust,  in  the  order  of  their  numbers,  with  the  films  toward  the  back  or  hinge  side  of 
the  boxes,  the  numbered  corners  uppermost,  and  the  lowest  numbers  in  front.  Wrap 
each  box  in  thick  paper,  secure  it  with  stout  twine,  and  mark  plainly  on  the  package 
the  numbers  of  the  plates  it  contains. 

AVith  each  batch  of  plates  prepared  for  observing  the  transit  at  least  two  test- 
plates  are  to  be  made,  one  near  the  beginning  and  one  near  the  end  of  the  operation, 
using  for  that  purpose  some  of  the  glass  not  selected  for  the  transit  work.     The  test- 
4  INS 


26  INSTRUCTIONS  FOE  OBSKK'VING  THE 

plates  are  to  be  dried  with  the  others  and  afterward  exposed  and  developed.     If  they 
prove  to  be  good  the  rest  of  the  lot  is  likely  to  be  equally  so. 


\\iii.—  KxrosriM-;  OK  IM,ATI:S  IN  THK 

The  sun's  image,  as  seen  on  the  reticule-plate  of  the  photoheliograph,  generally  has 
an  irregular  vibrating  motion  in  different  directions,  arising  from  various  causes.  As  the 
exposure  of  different  parts  of  the  plate?  lying  in  the  direction  of  motion  of  the  exposing 
>lide  is  not  simultaneous  but  successive,  any  movement  of  the  image  as  a  whole  will  pro- 
duce a  distortion  in  the  resulting  photograph  symmetrical  with  respect  to  its  vertical 
diameter,  and  inversely  proportional  to  the  velocity  of  the  slide.  Moreover,  the  limb 
itself  is  in  a  .Mate  of  constant  and  rapid  local  agitation,  which  in  some  conditions  of  the 
atmosphere  is  so  exaggerated  as  to  produce  the  boiling  or  Haming  appearance  familiar 
to  all  observers.  It  results  from  this  that,  quite  apart  from  any  distinctively  photo- 
graphic effect,  the  longer  the  exposure  the  larger  the  photograph  will  be,  and  that  it 
the  motion  of  the  slide  is  not  uniform  the  limb  on  the  side  where  the  velocity  was 
least  will  be  extended  more  than  the  other.  So  brief  is  the  time  of  exposure  that  these 
effects  are  indeed  minute,  but  it  cannot  lie  assumed  that  the\  are  in  all  cases  inappre- 
ciable. It  follows,  therefore,  that  a  given  exposure  is  more  advantageously  made  with 
a  wide  opening  of  the  slit  and  a  rapid  motion  of  the  slide  than,  with  a  narrow  opening 
and  a  slow  motion,  and  that  the  velocity  should  be  in  all  cases  as  nearly  uniform  as 
possible. 

It  is  expected  that  the  chief  astronomer  will  himself  expose  the  plates  on  the  day 
of  the  transit,  or,  at  least,  that  he  will  supervise  the  manipulation  of  any  person  to 
whom  this  duty  is  delegated,  and  see  that  such  assistant  lias  the  requisite  instruction 
and  previous  piactice.  The  movement  of  the  slide  should  be  as  rapid  as  can  lie  given 
with  ease  and  uniformity,  retaining  control  of  it  throughout,  and  avoiding  any  approach 
to  violence.  Toward  the  end  of  its  course  the  motion  should  be  slackened  so  as  not 
to  endanger  the  stops,  but  it  should  continue  until  the  slide  rests  against  them.  As  the 
regulation  of  exposure  depends  entirely  on  the  uniformity  of  this  movement,  it  should 
be  practiced  until  a  fixed  habit  is  acquired.  It  is  of  course  important  that  there  should 
be  no  great  difference  in  velocity,  whether  the  motion  is  from  east  to  west  or  from 
west  to  east.  To  increase  the  exposure,  widen  the  slit  by  separating  the  sliding  plates; 
to  diminish  it,  bring  them  nearer  to  each  other:  always  setting  them  so  that  the  center 
of  the  slit  shall  be  in  line  with  the  middle  pair  of  screws.  The  milled  nuts  must  be 
screwed  up  so  as  to  clamp  the  plates  securely  without  using  too  much  force. 

In  exposure  considerable  latitude  is  allowable;  indeed  the  appearance  of  the  sun's 
image  is  so  similar  with  very  different  exposures  that  the  real  importance  of  this 
element  is  liable  to  be  underrated.  The  planet  requires  less  exposure  than  the  sun's 
limb,  but  good  definition  of  the  latter  is  the  principal  end  to  be  attained.  If  with 
chemicals  in  good  order  and  proper  development  the  image  comes  out  reluctantly  and 
remains  very  thin,  or  if  there  is  any  material  falling  oil'  in  density  near  the  sun's  limb, 
the  exposure  i>  too  short.  To  find  the  correct  exposure,  begin  with  one  known  to  be 
sufficient,  and  gradually  lessen  it  until  signs  of  under-exposure  just  begin  to  appear. 
If  the  image  is  diMorted.  or  looks  as  if  the  plate-frame  were  out,  of  focus,  the  defect  is 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  27 

most  probably  due  to  flexure  of  the   mirror.     The  slightest  tension  arising  from 
improper  mounting,  either  of  lens  or  mirror,  will  make  itself  apparent  in  this  way. 

XIX.— THE  DEVELOPMENT. 

The  water  used  for  washing  the  plates  before  development,  and  the  developer  itself, 
should  have  a  temperature  of  at  least  60°  Fahr.;  90°  to  100°  is  still  better.  If  the 
weather  at  any  of  the  northern  stations  should  be  cold,  it  will  be  well  to  have  a  small 
supply  of  warm  water  at  hand  for  these  purposes. 

The  following  solutions  will  be  required  : 

Alcohol  and  tannin. 

Tannin  -  -     -     20  grains. 

Strong  alcohol  i  fluid  ounce. 

Water  i  fluid  ounce. 

Pyrogallic  solution. 

Pyrogallic  acid  3  grains. 

Water  -     i  fluid  ounce. 

This  solution  is  decomposed  by  keeping,  and  only  so  much  must  be  made  at  one 
time  as  can  be  used  immediately. 

Dilute  ammonia. 

Strong  ammonia  30  minims. 

Water    -  i  fluid  ounce. 

Bromide  solution. 

Bromide  of  potassium  20  grains. 

Water  i  fluid  ounce. 

To  these  may  be  added : 

Alkaline  citrate  solution. 

Citrate  of  ammonia  30  grains. 

Strong  ammonia  30  minims. 

Water  i  fluid  ounce. 

* 

The  dilute  ammonia,  bromide,  and  alkaline  citrate  are  to  be  transferred  for  use 
to  dropping-bottles,  so  conspicuously  labeled  that  they  can  be  easily  distinguished 
from  each  other  in  the  dark-room.  It  must  be  borne  in  mind  that  a  drop  from  one  of 
the  tubes  is  much  smaller  than  one  from  the  lip  of  a  bottle. 

Having  put  one  fluid  ounce  of  pyrogallic  solution  intooneof  the  small,  wide-mouthed 
bottles  issued  as  developing  glasses,  begin  by  treating  the  film  with  alcohol  and  tannin. 
One  principal  object  of  this  application  is  to  harden  the  substratum  of  albumen.  The 
solution  should  therefore  be  flowed  back  and  forth  over  the  plate  for  at  least  a  minute, 


28  INSTRUCTIONS  FOR  OBSERVING  THE 

and  be  returned  to  the  bottle  when  the  film  is  thoroughly  saturated.  The  manipulator 
is  next  to  be  wetted  and  applied  to  the  back  of  the  plate  unless  the  operator,  fearless 
of  stained  fingers,  prefers.to  dispense  with  it.  Wiisli  the  plate  under  the  tap,  or  with 
warm  water  if  necessary,  until  the  \vator  flows  smoothly  over  the  film.  Then  add  two 
drops  of  bromide  solution  and  two  drops  of  dilute  ammonia  to  the  pyrogallic  solution  in 
the  developing  glass  and  apply  it  to  the  plate,  keeping  it  in  gentle  motion  over  the  film  to 
p'r<  unote  equal  development.  The  image  should  appear  quickly  and  gradually  increa  sc 
in  strength.  Subsequent  additions  of  dilute  ammonia  are  to  be  made,  a  couple  of 
drops  at  a  time,  as  the  appearance  of  the  plate  may  indicate,  accompanying  every 
alternate  addition  with  an  equal  quantity  of  bromide. 

If  it  is  preferred  to  use  the  alkaline  citrate,  the  developer  is  to  be  prepared  at 
first  as  given  above,  and  the  subsequent  additions  will  be  of  the  alkaline  citrate  solu- 
tion only.  This  treatment  has  given  excellent  results  in  some  cases  and  is  provision- 
ally recommended. 

The  formula  given  above  is  offered  as  a  starting  point  for  such  modifications  as 
circumstances  may  require,  and  not  as  an  absolute  standard  of  universal  application. 
The  character  of  the  image  sought,  the  age  and  quality  of  the  emulsion,  etc.,  are 
varying  conditions  which  render  impracticable  the  adoption  of  any  rigid  rules.  The  end 
in  view  is,  by  means  of  a  developer  strong  in  pyrogallic  acid  and  weak  iu  ammonia  to 
bring  out  an  image  of  equal  density  throughout,  while  the  rest  of  the  [date  is  kept  clean 
by  a  sufficient  quantity  of  bromide,  or  of  bromide  and  citrate  of  ammonia.  A  greater 
strength  of  pyrogallic  acid  than  that  prescribed  can  hardly  be  required,  but  this  solution 
may  sometimes  be  advantageously  diluted.  If  the  image  come  out  slowly  it  must  be 
allowed  to  take  its  time,  and  not  be  forced  with  too  much  ammonia.  It  is  scarcely 
possible  to  produce  dense  fog,  but  a  veiled  image  is  usually  the  result  of  an  excess  of 
ammonia  Operators  accustomed  to  the  development  of  gelatine  plates  must  be  espe- 
cially cautious  in  this  respect. 

The  finished  photographs  must  belong  to  one  of  two  classes.  It  is  not  possible 
in  the  case  of  the  sun  to  obtain  an  outline  as  sharp  and  distinct  as  in  photographs  of 
the  moon  or  of  terrestrial  objects.  .But  the  nearest  possible  approach  to  such  a  defi- 
nite line  is  precisely  what  will  most  facilitate  measurement  of  the  plates,  and  this  must 
l>e  sought  as  being  of  much  greater  importance  than  the  attainment  of  any  special 
standard  of  density.  An  image  that  is  dull  and  blurred,  when  held  ov<!r  a  black  sur- 
face and  seen  as  a  positive  by  reflected  light,  and  that,  is  thin  and  ha/y  when  viewed 
as  a  negative  by  transmitted  light,  will  be  almost  worthless,  while  one  bright  and  clear 
as  seen  in  either  one  of  the  two  ways  will  be  valuable.  If,  therefore,  the  operator 
finds  it  easy  to  produce  a  sharp  and  clear  image  of  the  amhrotype  variety  he  may  do 
so,  taking  care,  of  course,  not,  to  discontinue  the  development  until  quite  certain  that 
it  is  einial  all  around.  Hut  if  there  is  from  any  canst'  a  tendency  to  discoloration  of 
the  film,  giving  a  dull  appearance  li\  reflected  light,  a  greater  degree  of  density  will 
be  requisite.  A  slight  veiling,  even,  is  not  very  injurious  if  the  image  is  clear  :lnd 
-tnuiLr  bv  transmitted  light.  No  effort  should  be  \\asted  in  striving  to  attain  an  un- 
iiece--;ir\  rli^rei-  ,,\'  density,  for,  although  it  can  hardly  be  too  great,  a  very  moder- 
ate density  will  suffice. 


TRANSIT  OP  VENUS,  DECEMBER  6,  1882.  29 

XX.— FIXING  AND  VARNISHING. 

As  soon  as  development  is  complete  wash  off  all  traces  of  the  developer  and  fix 
with  a  weak  solution  of  hyposulphite  of  soda.  Cyanide  of  potassium  must  not  be 

used. 

Fixing  solution.' 

Hyposulphite  of  soda,  J  oz.  to  i  oz. 

Water,     -     -  -     16  fluid  ounces. 

As  the  silver  bromide  dissolves  rapidly,  it  is  most  convenient  and  safest  to  pour  a 
little  of  the  fixing  solution  on  the  plate,  flow  it  about  until  the  film  is  clear,  and  finally 
dismiss  it  into  the  sink.  Then  wash  the  plate  thoroughly  and  set  it  in  a  rack  to  dry. 
After  the  lapse  of  sufficient  time,  several  hours  at  least,  varnish  in  the  usual  manner. 

XXL— BLISTERING  AND  LIFTING  OF  THE  FILM. 

If  the  film  separates  from  the  glass,  or  if  small  blisters  form  near  the  sun's  limb,, 
the  planet,  or  the  plumb-line,  the  plate  is  worthless,  and  no  precaution  must  be  neg- 
lected that  can  tend  to  prevent  so  great  a  misfortune.  A  substratum  of  albumen  that 
is  too  thick,  such  as  would  be  obtained  by  using  a  solution  materially  stronger  than 
that  of  the  formula  given  above,  may  be  imperfectly  coagulated  by  the  alcohol  and 
tannin,  and  thus  aggravate  the  evil  it  is  designed  to  prevent.  It  the  directions  already 
given  for  drying  the  albumenized  plates  thoroughly,  and  for  saturating  the  film  with 
the  solution  of  alcohol  and  tannin,  are  followed,  it  is  probable  that  no  trouble  will  be 
experienced;  but  should  a  tendency  to  this  defect  manifest  itself  during  the  preliminary 
practice,  special  caution  will  be  necessary  in  conducting  the  development  of  the  transit 
plates.  Too  much  ammonia  will  act  on  the  substratum  and  must  be  avoided.  The 
development  must  be  stopped  as  soon  as  the  least  allowable  density  is  reached,  and 
the  plate  must  be  fixed  by  pouring  on  the  hyposulphite  solution,  and  not  in  a  pan. 

XXII.— SPOTS  AND  OTHER  DEFECTS. . 

Small  circular  transparent  spots  may  appear  on  the  sun's  disk,  which,  if  numerous 
and  of  a  certain  size,  might  resemble  the  image  of  Venus  and  cause  time  to  be  lost  in 
measuring  the  plates.  The  emulsion  furnished  is  remarkably  free  from  any  tendency 
to  produce  these  spots.  If  they  appear  it  will  be  from  one  of  two  causes:  i.  Particles 
of  foreign  matter  in  the  emulsion;  2.  I'articles  of  dust  that  have  settled  on  the  plate 
after  coating,  especially  if  charged  with  chemical  substances,  such  as  might  arise  from 
solution  of  bichromate  of  potash  spilled  on  the  floor  of  the  dark-room.  The  former 
cause  may  be  removed  by  properly  filtering  the  emulsion;  the  latter  must  be  avoided 
by  cleanliness  and  greater  general  care. 

Certain  other  defects,  such  as  pin-holes,  crapy  lines,  &c.,  do  not  materially  detract 
from  the  value  of  the  plate,  but  every  operator  who  cares  for  his  reputation  will  never- 
theless seek  to  avoid  them. 

XXIIL— PREPARAT<  >RY  PRACTICE. 

To  familiarize  themselves  with  all  the  details  of  their  work  and  with  the  materials 
they  are  to  use,  the  photographers  will  begin  to  prepare,  expose,  and  develop  plates, 


:!(>  INSTIMVTIONS  FOR  OBSERVING  THE 

in  accordance  with  tin-  foregoing  instructions,  as  soon  as  possible  alter  arriving  at  their 
station,  ami  will  continue  such  exercise  until  the  chief  of  the  party  is  satisfied  that 
they  can  produce  solar  photographs  of  satisfactory  quality  with  ease  and  certainty. 
They  will  avoid  any  unnecessary  expenditure  of  materials,  and  will  take  care  that  a 
sufficient  supply  of  everything  is  reserved  for  the  operations  of  the  transit  day.  If 
the  bottles  of  emulsion  are  labeled  with  different  letters  it  signified  that  pyn>x\  lines  of 
slightly  different  character  have  been  used.  Test  the  contents  of  each  bottle,  and  if 
notable  differences  of  quality7  are  found  to  exist  among  them,  reserve  the  best  for  the 
transit  platrs,  or  mix  the  various  kinds  judiciously,  as  may  appear  to  be  preferable. 

It  has  been  already  remarked  that  the  person  who  is  to  expose  plates  during  the 
transit  must  acquire  a  fixed  habit  of  manipulating  the  exposing  slide,  and  that  unless 
an  approximately  uniform  motion  can  be  thus  obtained  there  will  be  no  means  of 
regulating  the  exposure.  Having  secured  a  tolerably  uniform  action  of  the  slide  by 
\vliateverpractice  may  be  requisite,  proceed  by  trial  according  to  the  instructions  given 
for  ascertaining  the  correct  exposure,  to  find  the  proper  opening  of  the  slit  when  the 
sky  is  quite  clear  and  the  atmosphere  in  the  most  favorable  state.  This  will  be  the 
least  opening  that  can  lie  used  at  any  time,  and  the  one  with  which  the  observation  of 
the  transit  will  be  commenced  if  the  weather  should  be  good. 

If  the  sun  is  obscured  bv  clouds  so  as  to  be  visible  for  a  few  minutes  only  during 
the  transit,  it  will  be  necessary  to  work  as  rapidly  as  possible.  The  manual  of  opera- 
tions prescribed  for  the  observation  of  the  transit  will  permit  plates  to  be  exposed  at 
intervals  of  only  a  few  seconds  if  every  one  is  perfectly  familiar  with  his  duties.  In 
order  that  each  may  be  prepared  to  perform  his  part  of  the  work  promptly,  the  whole 
party  must  be  drilled  from  time  to  time,  going  through  with  all  the  details  of  making 
the  record.  A: c.,  except  that,  plain  glass  will  be  used  instead  of  sensitive  plates,  and 
there  will,  of  course,  be  no  development. 

XXIV.—  I'KKI'AKATION   FOK  THE  TRANSIT. 

When  the  photographers  have  become  sufficiently  familiar  with  the  process  they 
are  to  use,  they  will  begin  to  prepare  a  stock  of  dry-plates  to  be  used  in  observing 
the  transit.  Kighteen  plate-boxes,  capable  of  holding  twelve  plates  each,  are  supplied 
to  each  party.  Seventeen  of  these  an;  to  be  filled  with  dry-plates,  leaving  one  box 
empty.  The  plates  are  to  In-  arranged  in  the;  boxes  in  the  order  of  their  numbers,  as 
already  directed,  and  the  boxes  must  be  so  marked  that  the  plates  can  lie  exposed  in 

( secutiv -der.      The   remaining  ylass   should  be  cleaned  and   albuineni/.ed,  to  lie 

used  with  wet  emulsion  in  case  of  emergency,  and  a  sntlicieiit  quantity  of  the  develop- 
ing solutions  for  fifty  or  more  plates  must  be  provided 

Everything  belonging  to  the  photoheliograph  must  be  in  correct  adjustment 
and  in  good  working  order,  especially  the  clock-work  of  the  heliostat,  which  must  be 
M  well  ri-ii-iilat.-d  that  it  can  be  left  to  itself  for  several  minutes  without  allowing  the 
sun's  imaire  to  get  too  far  from  the  center  of  the  plate.  See  that  the  mirror,  lens,  and 
reticule-plate  are  perfectly  dean,  and  remove  anv  dust  that  may  adhere  to  the  la -I 
named  by  wiping  it  with  chamois  leather.  If  artificial  light  is  required  for  reading 
the  chronometer  and  making  the  record,  as  it  probably  will  be,  an  orange-glass  lan- 
tern must  be  used. 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882. 


31 


XXV.— PHOTOGRAPHING  THE  TRANSIT. 

Photographs  are  to  be  taken  only  while  Venus  is  completely  within  the  limb  of  the 
sun.  When  the  chief  astronomer  decides  that  the  planet,  has  progressed  sufficiently  far 
upon  the  sun's  disk,  the  automatic  break-circuit  key  of  the  exposing  slide  will  be  put  in 
circuit  with  the  chronograph,  that  instrument  will  be  started,  the  members  of  the  party 
will  repair  to  their  respective  stations,  and,  after  locking  the  outer  door  of  the  photo- 
graphic house,  the  exposure  of  plates  will  begin.  The  services  of  four  persons  will  be  re- 
quired; if  the  party  consists  of  but  four,  their  duties  will  be  as  provided  in  what  follows: 

Tin'  tiKHixlimt  (ixtniitoiiicr  will  be  stationed  at  the  heliostat,  where  he  will  watch  the 
image  of  the  sun  on  the  target  of  the  exposing  slide,  and  keep  them  nearly  concentric 
.by  an  occasional  movement  of  the  tangent  screws.  The  image  must  never  be  allowed 
to  get  so  far  from  the  center  as  to  partly  uncover  the  black  disc  of  the  target.  He 
will  also  note  the  condition  of  the  sky,  and  give  prompt  warning  to  those  in  the  pho- 
tographic house  when  the  sun  is  obscured  by  clouds,  and  when  it  reappears.  He  will 
occasionally  see  that  the  chronograph  is  working  properly,  and  give  notice  when  it 
must  be  stopped  to  renew  the  paper.  And  finally,  he  will  read  the  barometer  and 
thermometer  as  directed  in  another  part  of  these  instructions. 

'f/ic  chief  astronomer  will  make  the  exposures  and  keep  the  record  in  the  following 
form : 

RECORD  OF  PLATES  EXPOSED  DURING  TI  IK  TRANSIT  OF  VENUS,  DEC  KM  HER  6, 1882 
At. .  ;   Chronometer 


Plate  ex- 
posed. 

Time  of  exposure  by  — 

Plumb-line 
pointer. 

Slide            Temp, 
moved.          Fahr. 

Remarks. 

Chronometer. 

Chronograph. 
h.         in.         s. 

JVff. 

//.             HI.             S. 

u 

> 

When  the  number  of  a  plate  is  called  he  will  enter  it  in  the  first  column,  and, 
taking  the  time  from  the  chronometer,  he  will  make  the  exposure  at  some  beat  of  the 
latter,  and,  after  giving  the  chronograph  signal,  or  "rattle,"  with  the  break-circuit  key, 
he  will  enter  the  chronometer  time  of  exposure  in  the  second  column.  The  time  re- 
corded by  the  chronograph  will  be  read  oft",  and  filled  in,  subsequently. 

In  the  column  headed  "plumb-line  pointer"  he  will  write  K.  or  W.,  as  the  case 
may  be.  The  pointer  should  be  frequently  reversed,  but  if  the  change  is  made  after 
the  exposure  of  every  plate,  the  plumb-line  will  probably  never  be  quite  at  rest.  A 
much  better  way  is  to  expose  the  plates  in  groups  of  six,  the  plates  of  a  group  follow- 
ing each  other  in  rapid  succession.  Then  reverse  the  pointer  and  give  the  plumb-line 
time  to  come  to  rest  before  exposing  the  next  group  of  plates. 

In  the  column  headed  "Slide  moved"  write  E.  or  W.,  according  to  the  direction 
of  that  motion  The  movement  should  be  alternately  eastward  and  westward,  and 
the  slide  must  always  be  left  resting  against  the  stops  that  limit  its  course. 


32  INSTRUCTIONS  FOE  OBSERVING  THE 


flu-  temperature  of  the  photographic  house  in  its  proper  column  at  intervals 
of  half  MM  liiuir,  or  more  frequently  if  the  change  is  rapid. 

In  the  column  tor  remarks  any  peculiarity  of  the  plates,  of  the  exposure  given, 
or  of  atmospheric  conditions,  and  any  other  circumstance  likely  to  iitl'ert  the  result, 
should  be  noted. 

When  tin-  chief  astronomer  desires  a  pause  in  the  work  to  follow  the  exposure  of 
any  plate  he  will  notify  the  photographer  who  changes  the  plates  of  his  intention 
In  -fore  giving  the  exposure,  so  as  to  avoid  unnecessary  handling1  of  the  plates. 

When  a  plate  has  been  developed  he  will  examine  it,  and,  with  the  advice  of  the 
photographers,  decide  whether  any  change  in  the  opening  of  the  slit  is  desirable. 

Our  at'  tlir  /il/ii/of/ni/il/i'i-x  will  put  the  plates  into  the  plate-frame,  and  remove  them 
after  their  exposure.  He  will  commence  operations  by  placing  the  empty  plate-box' 
on  the  right-hand  end  of  the  shelf  erected  for  that  purpose,  and  the  box  containing  the 
plates  that  bear  the  lowest  numbers  of  the  series,  beginning  with  No.  i,  at  the  left- 
hand  end  of  the  shelf.  When  directed  to  proceed  he  will  open  the  left-hand  box,  take 
out  the  nearest  plate,  which  should,  of  course,  be  No.  I,  carry  it  to  the  plate-frame, 
keeping  the  same  edge  of  the  plate  uppermost,  and  carefully  avoiding  any  contact 
with  the  plumb-line.  After  securing  the  plate  with  the  curved  spring  he  will  close  tin- 
plate-box,  and  then  call  the  number  of  the  plate,  reading  it  from  the  label,  which  will 
be  in  the  upper  left-hand  corner  of  the  plate-frame.  1  le  will  then  watch  the  back  of  the 
plait-  closely  to  see  that  the  sun's  image  is  not  too  near  the  edge  of  the  plate.  If  the 
distance  should  be  less  than  about  an  inch  he  will  so  report  to  the  chief  astronomer. 

When  the  image  appears  for  an  instant  on  the  plate  he  will  open  the  right-hand 
box,  take  the  plate  from  the  plate-frame,  carry  it  to  the  farthest  groove  in  the  box,  and 
then  close  the  box.  Without  waiting  for  orders  he  will  then  open  the  left-hand  box, 
take  out  the  nearest  plate  as  before,  and  so  on;  always  keeping  the  back  of  the  plate 
toward  himself,  and  the  numbers  uppermost,  and  always  replacing  the  plate  in  the 
farthest  unoccupied  groove  of  the  right-hand  box. 

When  the  right-hand  box  is  full  the  other  will  he  empty;  the  full  box  must  then 
have  a  gummed  label  pasted  over  the  hook  in  front,  so  that  it  cannot  be  easily  opened. 
and  must  be  put  in  some  place  specially  designated  for  the  reception  of  exposed  plates. 
Then  remove  the  empty  box  to  the  right-hand  end  of  the  shelf,  place  the  next  box  of 
the  series  to  the  left  of  it  as  before,  and  so  proceed  until  all  the  plates  are  exposed. 
To  avoid  mistakes,  the  boxes  should  be  arranged  beforehand  so  that  they  will  lie  taken 
up  in  consecutive  order. 

Tin  nflirr  photographer  "ill  develop  plates  during  the  whole  time  of  the  transit, 
beginning  with  the  one  first  exposed.  As  each  plate  is  tixed  and  washed  he  will  exam- 
ine it  carefully  to  see  if  the  exposure  is  correct,  and  will  call  the  attention  of  the  chief 
astronomer  to  any  alteration  that  maybe  required.  lie  will  then  develop  another 
plate,  always  taking  the  one  last  exposed.  The  grooves  in  tin*  boxes  belonging  to 
plates  taken  out  for  development  are  to  be  left  vacant,  and  not  filled  by  other  plates. 

If  the  sky  is  clear  the  groups  of  plates  exposed  should  be  equally  distributed 
over  the  whole  time  of  the  transit;  but  if  it  is  probable  that  the  entire  transit  \\ill  not 
be  seen,  a  considerable  number  of  photographs  should  be  secured  at  once,  reserving 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  33 

some  plates  to  be  used  if  the  weather  should  prove  better  than  was  anticipated.  If 
the  sun  is  visible  at  intervals  between  passing1  clouds,  everyone  must  be  at  his  station 
to  take  instant  advantage  of  every  opportunity  that  may  offer. 

The  sun  may  be  partially  obscured  by  thin  clouds  or  by  a  hazy  atmosphere,  so 
as  to  require  very  long  exposures.  After  the  slit  has  been  opened  as  widely  as  pos- 
sible the  exposure  can  be  still  further  increased  by  a  slower  movement  of  the  slide,  but 
good  results  can  hardly  be  expected  under  such  circumstances. 

XXVI.— DEVELOPMENT  AND  PACKING  OF  TRANSIT  PLATES. 

As  soon  after  the  transit  as  possible  the  photographers  will  begin  to  develop  the 
transit  plates,  and  will  continue  that  work  without  unnecessary  intermission  until  it  is 
completed.  After  drying  and  varnishing  the  plates  they  will  be  put  in  the  plate-boxes, 
and  kept  from  shaking  about  by  small  rolls  of  Joseph  paper,  long  enough  to  reach 
crosswise  over  the  tops  of  the  plates,  and  just  large  enough  to  hold  all  the  plates 
firmly,  without  too  much  force,  when  pressed  down  by  the  lid  of  the  box.  Two  such 
rolls  will  be  put  in  each  box.  The  boxes  are  then  to  be  covered  with  strong  paper 
and  tied  with  stout  twine. 

In  separating  the  plates  into  two  or  more  lots,  to  be  forwarded  at  different  times 
to  Washington,  the  plates  of  each  lot  should  be  selected  so  as  to  include  all  periods  of 
the  transit.  If  there  are  only  two  lots,  one  should  contain  all  the  even-numbered 
plates,  and  the  other  all  the  odd-numbered  ones. 

XXVII.— WET  EMULSION  PLATES. 

Wet  plates  may  be  prepai-ed  with  emulsion  if  any  accident  should  cause  the  loss 
of  a  great  part  or  all  of  the  dry -plates  when  it  is  too  late  to  replace  them.  A  glass 
plate  cleaned,  albunienized,  and  coated  with  emulsion,  as  already  directed,  is  to  be 
immersed  in  clean  water  contained  in  a  pan  or  dipping-bath  as  soon  as  the  film  is  set. 
When  the  water  flows  smoothly  over  the  film,  as  the  plate  is  lifted,  the  exposure  may 
be  made,  but  the  plate  may  remain  in  the  water  for  any  reasonable  length  of  time 
without  detriment.  Such  plates  are  developed  precisely  like  dry  ones,  omitting  the 
preliminary  treatment  with  alcohol  and  tannin.  They  develop  more  rapidly  than  dry- 
plates  made  from  the  same  emulsion. 

XXVIII.— BATH  WET  PLATES. 

Collodion  shrinks  greatly  in  drying,  but  when  once  dry  it  swells  but  little  if 
wetted  either  with  alcohol  or  water.  A  film  that  has  been  dried  before  it  is  exposed 
is  to  be  preferred,  therefore,  to  one  that  is  exposed  while  wet,  for  photographs  that 
are  to  be  accurately  measured.  And  dry-plates  can  be  exposed  so  rapidly  in  the 
photoheliograph  that  if  the  sun  should  be  visible  for  a  short  time  only  during  the 
transit,  a  most  important  advantage  would  be  gained  by  employing  them.  For  these 
and  other  reasons,  it  is  expected  that  they  will  be  used.  But  if  from  accidental  loss 
of  materials,  from  lack  of  experience,  or  from  any  other  cause,  the  photographers  are 
unable  to  prepare  and  develop  emulsion  dry-plates  successfully,  they  will  have  recourse 
to  the  usual  bath  process.  It  must  be  definitely  decided  in  advance  which  process 
will  be  used,  and  preparations  must  be  made  for  that  one  only. 

5    INS 


34  INSTRUCTIONS  POBOB8BBYQTG  TI1K 

If  a  resort  to  the  bath  wet  process  is  deemed  advisable,  the  services  of  at  least 
one  additional  photographer  must  lie  secured  if  possible.  With  three  or  four  photog- 
raphers it  will  probably  be  best  to  have  the  chief  photographer  collodionize  all  the 
plates  and  immerse  them  in  the  baths,  keeping  his  hands  clean.  The  other  photogra- 
phers will  withdraw  the  plates  from  the  baths  and  drain  them,  put  them  in  the  plate- 
frame,  and  call  their  numbers.  After  exposure  they  will  remove  the  plates  from  the 
plate-frame,  develop,  fix,  and  wash  them,  and,  finally,  put  them  in  the  drying  racks. 
Kadi  photographer  will  go  through  the  whole  course  of  those  manipulations  with 
every  plate  he  takes  from  the  bath,  and  the  different  operators  will  follow  each  othei 
as  rapidly  as  the  limited  accommodations  of  the  photographic  house  will  permit. 

The  best  collodion  is  one  that  gives  a  rather  hard  and  patchy  negative,  for  soft- 
ness, and  correct  rendering  of  light  and  shade,  are  rather  to  be  avoided  than  otherwise. 
The  following  formula  is  recommended  : 

Bromo-  iodized  collodion. 

m 

Alcohol     -  10  fluid  ounces. 

Ether  10  fluid  ounces. 

Iodide  ammonium  40  grains. 

Iodide  cadmium  60  grains. 

Bromide  ammonium  20  grains. 

At  least  three  silver  baths  will  be  required.  They  should  contain  40  grains  of 
nitrate  of  silver  to  the  ounce  of  water,  and  enough  nitric  acid  to  redden  blue  litmus 
paper  slowly.  On  removing  the  plates  from  the  bath  drain  them  as  thoroughly  as  is 
consistent  with  rapid  work,  and  wipe  their  backs.  Develop  with: 


Protosulphate  of  iron  1     ouncfe. 

Acetic  acid  No.  8-  1£  fluid  ounces. 

Water    -     -     -     -  -     20    fluid  ounces. 

These  proportions  may  be  varied,  however,  if  it  is  found  advisable  to  change 
them.  Fix  with  solution  of  cyanide  of  potassium  of  such  strength  as  to  clear  the  film 
rather  quickly. 

The  instructions  already  given  for  the  manipulation  of  dry-plates  will  be  followed 
so  far  as  they  are  applicable.  If  bath  wet  plates  have  been  exposed  in  the  photo- 
heHograph,  the  plate-frame  must  be  thoroughly  cleansed  from  all  traces  of  nitrate  of 
silver  before  dry-plates  are  put  into  it. 

x  x  i  x  .—  (  i  !•:  N  !•:  i  ;  A  i  ,  i  •  i  ;  i  •:<  '  A  r  T  i  <  >xs. 

Photographer*  who  are  accustomed  to  the  use  of  dry-plates  will  be  careful  from 
habits  already  formed,  but  those  who  have  practiced  only  the  usual  wet  process  must 
!"•  \ery  cautious  indeed  to  ;t  void  the  loss  of  plates  by  accidental  exposure  to  light. 

Plate-boxes  must  lie  opened  only  to  put  in  or  to  take  out  plates,  and  must  Ill- 
closed  again  a>  soon  as  possible.  The  drying-box  must  never  be  allowed  to  siand 

with  open  doon  to  avoid  the  inconvenience  of  opening  and  closing  them  frequentlv. 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  35 

Emulsion  bottles  and  vials  must  be  replaced  in  the  emulsion-chest  immediately 
after  using  them,  even  if  they  will  soon  be  wanted  again.  The  lid  of  the  chest 
should  be  secured  by  a  strap  or  otherwise,  so  that  it  cannot  be  left  open.  The  chest 
must  be  kept  locked,  with  the  key  in  some  specially  designated  place  and  not  in  the 
key-hole. 

When  filtering  emulsion,  and  while  making  or  exposing  plates,  the  outside  door 
of  the  house  must  invariably  be  locked. 

And,  finally,  the  various  manipulations  that  have  been  described  in  these  instruc- 
tions must  be  conducted  with  constant  and  scrupulous  regard  to  cleanliness.  It  is  only 
by  conscientious  attention  to  details  like  the  foregoing,  which  are  by  no  means  trivial, 
that  successful  results  can  be  confidently  anticipated. 

XXX.— THE  CHRONOGRAPH. 

The  speed  of  the  chronograph  is  governed  by  a  vibrating  spring  whose  normal 
rate  of  motion  is  one  hundred  and  thirty-two  vibrations  per  second.  In  regulating  it, 
the  time  of  revolution  of  the  cylinder  must  be  made  correct  within  about  two  per 
centum  by  moving  the  sliding  weight  near  the  root  of  the  spring,  and  then  the  final 
adjustment  can  be  effected  while  the  instrument  is  running  by  slightly  loosening  or 
tightening  the  capstan-headed  screw  confining  the  cheeks  between  which  the  spring  is 
held. 

The  speed  of  the  train  is  so  great  that  to  avoid  detrimental  friction  all  the  pivots 
must  be  frequently  oiled,  but  especially  the  escape- wheel  pivot,  and  those  near  it. 
Weights  of  one  hundred,  fifty,  twenty-five,  and  twenty-five  pounds  are  furnished  with 
the  instrument,  but  in  ordinary  summer  weather  one  hundred  pounds  drives  it  well. 
In  winter,  a  little  more  may  be  needed.  The  weight  should  have  space  to  fall  three 
feet.  This  suffices  to  run  the  instrument  two  hours,  and  if  it  is  wound  when  a  fre"sh 
sheet  of  paper  is  put  on  the  cylinder,  it  will  not  require  winding  again  till  the  paper 
is  changed. 

XXXI.— TIME  OBSERVATIONS  AND  CHRONOMETER  COMPARISONS. 

Should  the  station  not  be  in  telegraphic  communication  with  a  fixed  observatory 
from  which  local  time  is  received,  two  azimuth  stars  above  the  pole,  two  below  the  pole, 
and  six  or  eight  time  stars  should  be  observed  with  the  transit  instrument  on  every 
night  when  it  is  practicable.  One-half  the  observations  of  each  class  should  be  made 
with  clamp  east,  the  other  half  with  clamp  west.  Should  the  observer  be  able  to  get 
his  local  time  from  a  fixed  observatory,  his  own  determinations  may  be  omitted  when 
not  necessary  to  the  success  of  the  expedition.  They  must,  however,  be  carefully 
attended  to,  so  far  as  may  be  required,  either  for  the  determination  of  his  own  longi- 
tude, or  for  comparing  his  local  time  with  that  of  other  parties  in  the  neighborhood. 
In  any  case,  enough  observations  must  be  made  to  determine  the  azimuth  of  the  photo- 
heliograph  and  detect  any  changes  that  may  occur  in  it. 

All  chronometers  at  the  station  must  be  compared  daily,  when  they  are  wound, 
unless  the  local  time  and  longitude  are  so  well  determined  that  no  interest  attaches 
to  their  running.  It  will  sometimes  be  necessary  to  carry  one  of  them  about, 


3G  INSTRUCTIONS  FOR  OBSERVING  THE 

but  the  others  should  never  be  moved  when  it  can  be  avoided.  Kvery  care;  should  be 
taken  to  keep  them  at  as  uniform  a  temperature  as  possible,  and  the  degree  of  heat  to 
which  they  are  exposed  should  be  noted  and  recorded  three  or  four  times  a  day. 

XXXII.— EXCHANGE  OF  TIME  WITH  OTHER  PARTIES. 

Should  any  opportunity  offer  for  the  comparison  of  local  time  or  chronometers 
with  parties  from  other  countries,  it  must  be  improved.  In  such  case,  the  observer 
must  be  careful  to  keep  a  complete  copy  of  the  comparison,  and  to  assure  himself  that 
he  has  all  the  data  necessary  for  determining  the  difference  of  longitude  between  the 
stations  compared. 

XXXIII.— LATITUDE  AND  LONGITUDE  OF  STATION. 

The  latitude  of  the  station  must  be  determined  with  the  meridian  instrument, 
used  as  a  zenith  telescope,  and  not  less  than  thirty-six  observations  should  be  made 
upon  at  least  twelve  pairs  of  carefully-selected  stars. 

Special  attention  must  be  paid  to  getting  the  true  longitude  of  the  station,  but 
the  best  method  of  doing  this  will  depend  upon  circumstances.  If  the  station  is  in  a 
region  covered  by  an  accurate  trigonometrical  survey,  or  if  it  is  in  telegraphic  com- 
munication with  a  fixed  observatory,  the  determination  of  its  longitude  will  be  com- 
paratively easy.  In  any  case,  the  observer  must  be  on  his  guard  against  depending 
upon  a  single  result.  If  accurately  known  trigonometrical  points  are  available,  the 
position  of  the  station  must  not  be  determined  from  a  single  one  of  them,  but  from  at 
least  three,  whenever  possible.  If  telegraphic  signals  are  exchanged  with  a  fixed 
observatory,  the  exchange  should  be  continued  through  not  less  than  three  evenings. 
At  places  where  neither  the  trigonometrical  nor  the  telegraphic  method  is  available, 
recourse  must  be  had  to  occultations  and  moon  culminations.  In  observing  the  latter, 
care  should  be  taken  that  the  number  of  observations  before  and  after  full  moon  are 
nearly  equal,  and  that  in  each  class  about  as  many  observations  are  made  by  the 
assistant  astronomer  as  by  the  chief  of  party.  Instructions  respecting  occultations 
are  given  in  section  XXXV. 

XXXIV.— THE  EQUATORIAL  TELESCOPE. 

At  stations  where  it  is  necessary  to  observe  occultations,  the  mounting  of  the 
equatorial  must  be  commenced  as  soon  as  possible,  taking  precedence  of  that  of  the 
photographic  piers.  The  site  selected  must  be  such  that  the  instrument  commands  a 
good,  view  of  the  eastern  and  western  sky,  to  within  live  degrees  of  the  horixon  if 
practicable. 

Caution. — The  shade  glasses  supplied  with  the  double-image  micrometer  are  so 
constructed  that  they  can  be  employed  tolerably  >afely  upon  a  bright  sun  with  the 
full  aperture  of  the  telescope,  but  as  a  matter  of  precaution,  they  should  lie  wanned 
a  little  before  using  them,  and  the  telescope  should  never  be  left  pointed  at  the  sun  a 
moment  longer  than  is  necessary.  With  the  shades  supplied  for  the  Airv-I  luvgenian 
eye-pieces  it  is  quite  otherwise.  If  the  sun's  rays  from  the  full  aperture  of  the  tele- 
scope are  allowed  to  ].;i>s  through  one  of  them  with  imdiminished  force,  it  will  split  at 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  37 

once,  to  the  great  danger  of  the  observer's  eye.     For  that  reason  they  must  never  be 
used  apart  from  the  Herschel  solar  prism. 

XXXY.— OCC  ULTATIONS. 

At  stations  whose  longitude  is  not  otherwise  determined,  all  visible  occultations 
of  stars  by  the  moon  which  occur  during  the  stay  of  the  party,  must  be  carefully 
observed.  To  facilitate  this  work,  the  instants  of  the  emersions  which  happen  after 
the  full  moon  may  be  computed  in  advance. 

From  the  time  the  new  moon  first  becomes  visible  until  her  full,  she  is  to  be  care- 
fully watched  with  the  telescope  to  see  what  stars  will  be  occulted.  These  can  be 
recognized  from  thirty  to  sixty  minutes  beforehand  by  remembering  that  the  course  of 
the  moon  is  nearly  at  right  angles  with  the  line  joining  her  cusps,  and  that  she  moves 
nearly  her  own  diameter  in  an  hour.  Whenever  there  is  a  chance  of  seeing  an  occul- 
tation,  a  map  of  the  relative  positions  of  the  moon  and  the  stars  in  its  neighborhood 
must  be  made;  and  if  the  occultation  is  actually  observed,  the  exact  point  of  the 
moon's  limb  at  which  the  star  disappeared  must  be  noted  on  the  sketch.  Nothing 
must  be  recorded  as  an  immersion  or  emersion  except  the  actual  sudden  and  distinct  dis- 
appearance or  re-appearance  of  the  star  at  the  moon's  limb.  If  the  star  is  lost  in  the 
moon's  rays  at  that  moment,  the  fact  should  be  stated.  Every  observation  must  also 
specify  the  maker's  name  and  number  of  the  time-piece  employed,  and  whether  the 
occultation  was  well  observed,  and  if  not,  what  amount  of  uncertainty  attaches  to  it. 

Great  care  must  be  taken  to  guard  against  errors  of  ios  in  the  record,  and,  to 
this  end,  it  will  be  well  to  have  an  assistant  call  aloud  every  tenth  second,  "o,"  "10," 

"20,"  etc. 

The  chronograph  may  be  used  in  observing  occultations,  but  in  that  case  a  frac- 
tion of  a  second  will  be  required  for  the  observer  to  become  conscious  of  the  phe- 
nomenon, and  to  give  the  signal,  and  this  interval  must  always  be  estimated  by  the 
observer,  and  recorded  in  the  memorandum-book.  But,  the  chronograph  should  never 
be  trusted  to  exclusively,  and,  when  used,  either  the  observer  himself  or  his  assistant 
should  note  the  chronometer  time  of  the  occultation,  or  of  the  signal  with  the  key. 

XXXVI.— GENERAL  INSTRUCTIONS  RESPECTING  OBSERVATIONS  OF  CONTACTS. 

The  first  question  which  the  intending  observer  of  contacts  has  to  consider  is 
whether  the  appliances  at  his  disposal  and  the  circumstances  in  which  he  is  placed 
will  permit  of  his  making  observations  of  astronomical  value.  If  they  do,  especial 
pains  and  minute  attention  must  be  devoted  to  all  the  necessary  preparations.  The 
following  is  an  outline  of  the  general  plan  of  operations: 

Determination  of  time. — The  most  essential  requirement  is  that  the  observer  shall 
have  the  means  of  determining  his  local  time  within  at  least  one  or  two  seconds  At 
fixed  observatories  there  need  be  no  difficulty  in  this  respect.  For  the  benefit  of 
observers  at  other  points  it  is  intended  to  make  arrangements  with  the  Western  Union 
Telegraph  Company  to  transmit  time-signals  from  the  Naval  Observatory  to  every 
part  of  the  country.  Individual  observers  who  receive  their  time  in  this  way  should 
communicate  with  the  authorities  at  the  nearest  telegraph  station,  and,  in  the  event  of 


:5S  INSTRUCTIONS  FOR  OBSERVING  THE 

any  doubt,  address  the  Superintendent  of  the  Xaval  Observatory,  Washington,  on  the 
subject.  Detailed  information  and  instructions  for  receiving  and  understanding  the 
time-signals  will  be  sent  to  all  who  desire  it  in  advance  of  the  transit. 

Sue  and  quality  of  telescope. — The  aperture  df  telescope  to  be  preferred  in  the 
observations  is  from  5  to  6  inches.  In  order  that  all  observations  may  be  as  nearly  as 
possible  comparable  with  those  made  in  the  Southern  hemisphere,  it  is  recommended 
that  observers  with  telescopes  exceeding  6  inches  in  aperture  reduce  the  aperture  to  6 
inches  in  observing  all  the  contacts.  Apertures  as  small  as  4  inches  may  be  used 
without  seriously  detracting  from  the  accuracy  of  the  observation.  Below  4  inches 
the  value  rapidly  diminishes,  and  3  inches  may  be  regarded  as  the  smallest  with 
which  observations  of  real  value  can  be  made.  It  is  important  that  the  objective 
should  be  of  good  quality,  forming  round,  neat  images  of  stars',  with  a  power  as  high 
as  200.  To  test  the  objective,  alternately  push  the  eye-piece  in  and  draw  it  out  so 
that  the  star  shall  present  the  appearance  of  a  disk  of  light.  If  the  objective  is  good, 
this  disk  will  be  round  and  of  uniform  brilliancy  ;  if  the  disk  is  irregular  in  outline, 
with  permanent  bright  or  dark  regions  in  it,  it  shows  the  telescope  not  to  be  good. 

Magnifying  power. — The  eye-piece  should  have  a  magnifying  power  not  less  than 
150  nor  much  more  than  200.  Between  these  limits  the  choice  may  be  regarded  as 
indifferent. 

Mounting. — An  equatorial  mounting  is  nearly  indispensable  to  an  accurate  observa- 
tion. Observers  practiced  in  the  use  of  an  altazimuth  mounting  may  possibly  make  an 
observation  with  one  of  that  class,  but  they  must  be  able  to  keep  an  object  in  the 
center  of  the  field.  A  clock-motion  is  desirable,  though  not  indispensable.  If  there 
is  no  clock-motion  the  telescope  must  be  firmly  mounted,  and  the  observer  must  be 
well  practiced  in  moving  the  eye-piece  steadily  with  his  hand  so  as  to  keep  an  object 
in  the  center  of  the  field. 

Micrometer. — A  regular  filar  micrometer  will  not  be  of  any  use  as  an  instrument  of 
measurement,  but  spider-lines  of  some  sort  are  desirable  for  the  double  purpose  of 
insuring  an  accurate  adjustment  of  focus  and  of  estimating  the  brilliancy  of  the  sun's 
disk.  If  the  telescope  is  not  supplied  with  a  micrometer  the  observer  should  have  a 
positive  eye-piece,  in  the  focus  of  which  he  should  insert  a  spider-line  or,  better  yet,  if 
he  is  able  to  do  it,  a  pair  of  spider-lines  at  such  a  distance  that  the  angle  between 
them  shall  be  i"  or  2"  of  arc.  In  a  6-foot  telescope  the  required  distance  will  be  about 
.ji,  of  an  inch.  The  observer  should  find  by  previous  trials  on  the  sun  and  stars  the 
exact  point  when  the  spider-lines  are  in  the  focus  of  the  objective  so  as  to  insure  their 
being  in  proper  position  on  the  day  of  the  transit.  This  point  may  be  indicated  by  a 
mark  on  the  eye-piece. 

Shcuh  (/liivscs. — The  common  sun-shades,  consisting  of  a  single  piece  of  thick 
•jlax,  are  very  apt  to  split,  and  thus  endanger  the  observer's  eve,  if  the  rays  of  a 
bright  sun  are  concentrated  upon  them  by  an  objective  of  larger  aperture  than  two 
inches  for  ;I  local  distance  of  thirty  inches,  or  three  inches  for  a  focus  of  five  or  six 
feet.  My  making  the  shade  of  three  thicknesses  of  -lass,  the  piece  next  the  eye 
being  thickest  and  darkest  in  color,  the  other  pieces  being  successively  thinner 
and  lighter  in  color,  and  all  being  lifted  loosely  into  their  cell  so  that  they  can 


TRANSIT  OF  VENUS,  DECEMBER  6,   1882.  39 

expand  freely,  it  will  be  possible  to  use  with  safety  an  aperture  of  five  inches  upon 
a  telescope  of  six  feet  focus.  It  is,  however,  recommended  that,  wherever  possible, 
some  other  means  of  diminishing  the  "sun's  light  be  employed.  Silvering  the  object- 
•ive  might  be  recommended,  except  for  the  possibility  of  cutting  off  too  much  light 
in  a  hazy  atmosphere.  The  polariscopic  eye-piece  is  commended  for  its  convenience 
in  use.  If  the  observer  cannot  avail  himself  of  it,  a  diagonal  eye-piece  with  a  re- 
flector of  plain  unsilvered  glass  is  recommended.  In  such  an  eye-piece  the  reflector 
is  placed  a  little  in  front  of  the  focus  at  an  angle  of  45°  with  the  axis  of  the  telescope. 
Being  unsilvered,  92  per  cent,  of  the  light  passes  through  it,  and  should  be  permitted 
to  leave  the  telescope  through  an  opening  so  as  not  to  heat  the  air  or  the  reflector. 
The  remaining  8  per  cent,  of  the  light  is  reflected  from  the  two  surfaces  of  the  glass. 
In  order  that  these  two  systems  of  reflected  rays  may  not  cause  confusion,  the  glass 
should  be  ground  wedge-shape,  and  so  arranged  that  only  the  reflection  from  the  first 
surface  may  reach  the  eye.  Since  4  per  cent,  of  the  sun's  light  will  in  nearly  all  cases 
be  too  great  for  the  eye,  the  observer  should  also  be  provided  with  shade-glasses  to  still 
further  diminish  it.  A  neutral  tint  is  to  be  preferred  for  all  such  glasses. 

Day  of  the  Transit. — It  is  essential  that  every  observer  intending  to  make  a  really 
accurate  observation  should  have  little  else  to  attend  to  during  at  least  an  hour  or  two 
before  the  first  contact  he  is  to  observe,  and  should  be  entirely  free  from  visitors  and 
inquirers.  The  points  to  be  particularly  attended  to  are  .the  firmness  of  the  telescope, 
his  ability  to  move  it  in  such  a  way  as  to  keep  any  required  part  of  the  sun's  limb 
steadily  in  the  center  of  the  field,  and  the  accuracy  of  the  focal  adjustment.  A  mere 
estimate  of  an  accurate  focus  about  the  time  of  observation  should  not  be  trusted  to 
if  it  can  be  avoided,  because  the  eye  itself  is  liable  to  change  its  accommodation  in 
this  respect.  The  surest  course  is  to  have  a  pair  of  spider-lines  previously  fixed  in  the 
astronomical  focus  and  to  adjust  the  eye-piece  so  that  these  lines  shall  be  sharply  defined 
on  the  sun's  disk.  The  observer  can  then  be  certain  that  his  focus  is  right  so  long  as 
the  definition  of  the  wires  continues  good. 

The  degree  of  brilliancy  of  the  sun's  disk  as  seen  by  the  eye  is  to  be  particularly 
attended  to.  It  was  recommended  by  the  Paris  International  Conference  that  the  disk 
should  be  darkened  to  the  point  at  which  a  pair  of  spider  lines  i"  apart  could  just  be 
seen  distinctly  separated.  But  as  this  test  may  not  suffice,  and  as  the  observer  may 
find  insuperable  difficulty  in  fixing  the  wires  so  close  to  each  other,  some  other  tests 
should  be  employed.  We  may  lay  down  limits  as  follows: 

I.  If  the  brilliancy  of  the  disk  is  such  as  to  be  at  all  unpleasant  to  the  eye,  or  if 
there  is  any  appearance  of  glare*  surrounding  the  sun's  limb,  then  the  light  is  too 
bright  and  must  be  diminished. 

II.  If  there  is  any  difficulty  in  seeing  the  limb  well  and  brightly  defined,  then  the 
light  is  too  faint.     Perhaps  a  good  rule  will  be  to  shade  off  the  light  to  such  a  degree 
that  with  the  center  of  the  sun  in  the  center  of  the  field,  the  whole  field  is  as  bright 
as  the  observer  finds  it  not  unpleasant  to  look  at  continuously,  and  yet  not  so  bright 
as  to  render  the  mottling  of  the  photosphere  indistinct      Then,  since  the  sun's  limb  is 

*  Tins  word  is  here  used,  not  in  the  s*  use  of  a  general  atmospheric  illumination,  but  in  the  sense  of  such  a  reful- 
gence as  lei  produce  ;m  ap]ii  arance  ot;  imlist  inetm  ss  of  outline  through  that  excitation  of  the  eye  itself  known  as 


40  INSTKMTIONS  FOROr.SKKYINC   TIIK 

less  than  half  as  bright  as  the  center  of  its  disk,  it  may  be  presumed  that  the  latter 
will  be  about  of  the  right  shade.  But  it  must  always  be  remembered  that  the  slightest 
glare  indicates  too  great  a  brilliancy. 

Vft  another  guiding  rule  will  be  that  the  most  distinct  arid  easy  view  of  the  sun's 
limb  is  to  be  aimed  at. 

ilrtcrnnl  contuct. — To  make  a  really  good  observation  of  this  contact  two  conditions 
are  essentially  necessary  in  addition  to  all  which  have  been  described.  The  observer 
must  have  had  some  previous  practice  in  observing  first  contacts,  and  he  must  know 
exactly  where  to  look  for  the  contact. 

The  first  condition  can  be  well  fulfilled  by  the  artificial  transit  of  \renus  apparatus, 
of  which  it  is  intended  to  have  one  or  more  in  Washington  available  for  observers. 

Fur  the  second  condition  it  is  essential  that  the  observer  shall  have  the  means  of 
setting  the  spider-lines  in  the  field  of  view  to  any  required  angle  of  position.  Within 
the  United  States  the  first  contact  will  occur  at  a  point  of  the  limb  found  by  measuring 
147°  from  the  north  point  towards  the  east.  The  spider-lines  should  be  set  at  right 
angles  to  that  radius  of  the  solar  disk  which  terminates  at  this  point  of  the  limb.  Then 
cutting  off  a  segment  of  the  disk  by  the  spider-line,  first  contact  will  be  seen  in  the 
middle  of  this  segment.  The  tabular  time  of  first  contact  at  any  point  on  the  earth's 
surface  may  be  found  within  a  minute  by  subtracting  21  minutes  from  the  time  of 
internal  contact.  The  interval  between  these  contacts  may  be  found  with  yet  greater 
precision  from  the  tables  in  the  American  Ephemeris  for  1882.  The  tabular  Green- 
wich time  of  internal  contact  may  be  taken  at  sight  from  the  proper  map  accompa- 
nying this  paper.  Within  the  United  States  the  tabular  time  of  first  contact  may  be 
regarded  as  20  hours  55  minutes  Washington  time.  In  civil  reckoning  this  is  five 
minutes  before  nine  a.  m.  Owing,  however,  to  the  errors  of  the  tables,  which  obser- 
vations of  the  transit  will  help  us  in  correcting,  it  is  quite  possible  that  first  contact 
will  occur  a  large  fraction  of  a  minute  earlier  than  the  predicted  time.  To  allow  for 
this  possible  error,  it  is  recommended  that  the  observer  begin  to  look  exactly  one 
minute  before  the  computed  time.  The  following  is  a  specimen  of  part  of  the  com- 
putation which  the  observer  should  make  by  the  aid  of  the  chart  in  order  to  know 
when  to  begin  looking.  Suppose  the  place  to  be  Cincinnati.  We  find  from  the  chart : 

ll.         U).          8. 

Greenwich  mean  time  of  internal  contact .      2   24  52 

Longitude  of  Washington 5     8  12 


Washington  mean  time 21    16  40 

Induction  to  external  contact 21    15 


Washington  time  of  tabular  contact 20  55  25 

Washington  time  to  begin  looking 20  .54   25 

The  observer  should  avoid  looking  before  this  time  in  order  not  to  fatigue  his  eve. 

The  time  to  be  noted  is  that  at  which  the  notch  made  bv  the  advancing  planet 
tir>t  liei-Mines  visible.  The  observer  may  have  to  wait  a  few  seconds  to  be  sure  that 
what  he  M6S  is  n-ally  a  permanent  notch,  but,  the  time  to  be  given  is  that  when  it  was 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  41 

first  certainly  seen.     If  he  did  not  catch  the  first  moment  when  he  could  see  it,  that 
fact  must  be  stated. 

Internal  contact. — Owing  to  the  importance  of  this  observation  and  the  necessity 
of  special  attention  to  it,  it  is  recommended  that  the  observer  have  little  else  to 
attend  to  during  the  2 1  minutes  between  it  and  external  contact.  It  is  now  believed 
that  measures  of  the  cusps  with  a  double-image  micrometer  should  not  be  undertaken 
during  this  interval,  owing  to  their  fatiguing  the  eye  and  distracting  the  attention  of 
the  observer. 

It  is  essential  that  the  observer  should  allow  his  eye  nearly  perfect  repose  for 
several  minutes  before  the  contact.  It  is  quite  right  and  proper  that  he  should  take  a 
general  view  of  the  phenomenon  at  short  intervals,  and  note  the  appearance  presented 
by  the  outline  of  the  planet,  but  he  should  not  exercise  his  eye  and  attention  in  endeav- 
oring to  make  any  difficult  observation. 

His  serious  attention  will  be  first  required  some  two  minutes  before  the  expected 
time  of  contact.  There  is  every  reason  to  believe  that  the  entire  outline  of  the  planet 
will  then  be  visible,  that  portion  not  on  the  solar  disk  being  bounded  by  a  fine  line  of 
light,  supposed  to  be  due  to  the  refraction  of  the  atmosphere  of  the  planet.  Indeed, 
this  line  may  be  visible  from  the  first  moment  of  the  planet's  appearance,  and  the 
changes  which  it  undergoes  in  the  relative  brilliancy  at  different  points  will  be  a  sub- 
ject of  great  scientific  interest.  Although  observers  of  accurate  contacts  must  guard 
against  fatiguing  their  eyes  by  minute  observations  on  this  arc  of  light,  observers 
who  have  not  the  appliances  for  making  the  best  observations  of  contact  might  well 
devote  themselves  to  its  careful  study. 

One  of  the  great  difficulties  in  the  observation  of  internal  contact  will  be  to 
avoid  confusing  this  line  of  light,  which  may  grow  brighter  as  contact  approaches, 
with  the  direct  light  from  the  sun's  limb,  which  will  be  seen  after  contact.  The 
distinction  of  the  two  is  a  matter  of  judgment  which  must  be  left  with  the  observer. 
In  what  follows  we,  for  the  most  part,  make  abstraction  of  this  appearance,  describing 
phenomena  as  if  it  were  not  present,  and  the  observer  must  in  like  manner  seek  to 
observe  as  if  it  were  not  present. 

The  moment  to  be  observed  as  that  of  true  internal  contact  is  when  the  limb  of 
Venus  is  exactly  tangent  to  that  of  the  sun.  It  is,  however,  found  by  experience  that, 
although  easy  to  think  of  this  tangency,  it  is  difficult  to  observe  it  with  the  requisite 
precision,  owing  to  the  imperfection  of  vision,  and  especially  to  the  irradiation  produced 
by  the  earth's  atmosphere  and  by  any  imperfections  in  the  telescope.  The  phenomena 
to  be  really  observed  are  defined  as  follows  in  the  instructions  of  the  International 
Conference  held  at  Paris  in  1 88 1 : 

"At  ingress  the  moment  to  be  noted  is  that  at  which  the  observer  sees  for  the  hist  time  an 
evident  and  persistent  discontinuity  in  the  apparent  limb  of  the  sun  near  the  point  of  contact  with 
Venus." 

"At  egress  the  moment  of  the  first  appearance  of  a  well-marked  and  persistent  discontinuity 
in  the  illumination  of  the  apparent  limb  of  the  sun  at  the  point  of  contact." 

However  well  these  definitions  may  apply  to  the  actual  phenomena,  they  are  not 
sufficient,  withoi;t  further  explanation,  to  enable  the  observer  to  know  what  is  contact 
6  INS 


42  INSTRUCTIONS  FOR  OBSERVING  THE 

under  all  circumstances.  Indeed,  the  Conference  itself  added  a  number  of  instructions 
of  what  was  to  be  looked  for  under  special  circumstances.  For  these  instructions  the 
following  are,  however,  substituted  by  the  American  Commission. 

We  must  first  remember  that  just  before  internal  contact  at  ingress  the  sun's 
limb  will  be  broken  off  by  the  advancing  planet,  and  that  portion  which  is  visible 
near  the  point  of  contact  will  present  the  appearance  of  two  fine  sharp  horns,  the 
points  of  which  will  slowly  approach  each  other.  The  moment  of  true  internal  con- 
tact is  evidently  that  at  which  these  points  exactly  meet.  But  since  they  cannot  be 
seen  by  the  eye  to  meet  until  the  completed  line  of  light  becomes  thick  enough  to  be 
seen,  the  observer  must  not  expect  to  see  the  thread  of  light  actually  complete  until 
after  the  contact  has  passed.  As  a  general  rule,  therefore,  he  must  note  what  is  to 
be  seen  just  before  this  thread  of  light  becomes  evidently  complete.  What  he  sees 
will  depend  very  largely  upon  the  clearness  and  steadiness  of  the  air.  The  most 
favorable  circumstances  for  observing  true  contact  are  those  in  which  the  cusps  appeal- 
steady  and  sharply  defined  against  the  black  background  of  the  sky.  There  will 
then  be  little  difficulty  in  catching  the  moment  at  which  they  are  first  about  to  meet, 
which  will  be  that  of  true  contact. 

In  most  cases,  however,  especially  if  the  sun  is  low,  the  outline  of  the  cusps  will 
be  wavy  and  serrated,  their  ends  will  be  more  or  less  rounded  instead  of  being  sharp, 
and  their  outline  will  be  continually  changing  in  consequence  of  the  apparent  undu- 
lating motion  produced  by  the  atmosphere.  The  greater  this  vibratory  motion  and  the 
more  the  cusps  are  blunted  the  more  difficult  it  will  be  to  catch  the  moment  of  true  con- 
tact. The  following  rules  are  then  to  be  borne  in  mind  by  the  observer:  So  long  as  the 
dark  region  between  the  cusps  which  connects  the  black  disk  of  Venus  with  the  black 
sky  outside  the  sun  retains  its  full  darkness,  without  any  apparent  motion  or  undula- 
tion going  across  it,  so  long  contact  has  not  occurred;  and  this  although  the  planet 
may  seem  entirely  within  the  true  outline  of  the  sun.  It  would  be  well  for  the 
observer  to  have  an  assistant  at  the  chronometer  to  whom  he  can  from  time  to  time 
call  out  the  words  "not  yet."  The  assistant  should  write  down  the  second  by  the 
watch  or  chronometer  at  which  the  observer  commenced  to  pronounce  these  words. 

Instead  of  the  cusps  uniting  into  a  fine,  steady  line  of  light,  the  observer  may  at 
a  certain  moment  begin  to  see  an  undulating  motion  extending  all  the  way  across  this 
dark  space.  He  will  soon  after  see  that  this  motion  is  due  to  the  continually  increas- 
ing line  of  light,  which  is  broken  into  threads  and  waves  bv  atmospheric  undulation. 
From  and  after  the  time  that  this  undulation  is  permanently  seen  contact  is  certainly 
passed.  It  would  be  well,  when  it  is  first  fully  recogni/ed,  that  the  observer  should 
call  out  "past"  to  his  assistant,  who  should  note  the  time  at  which  the  word  is  spoken. 
If  he  has  made  no  mistake  in  his  estimates  the  time  of  contact  will  be  limited  between 
the  last  moment  at  which  "not  yet"  was  spoken  and  the  first  moment  at  which  "past" 
was  pronounced. 

In  tlie  event  of  the  cusps  appearing  much  rounded,  the  further  Venus  appears 
inside  the  disk  of  the  sun,  as  completed  in  the  imagination  by  continuing  its  outline 
ai-ro-s  the  dark  region,  the  more  careful  must  the  observer  be  fo  catch  the  first  line  of 
true  sunlight  extending  across.  It  may  be  assumed  that  if  the  seeing  is  at  all  good  the 
undulating  light  of  the  sun's  limb  will  be  clearly  recognized  in  a  very  few  seconds  after 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  43 

the  true  time  of  contact.  On  the  other  hand,  he  must  be  on  his  guard  against  mistaking 
some  slight  haziness  around  the  point  of  contact  for  the  appearance  of  true  sunlight.  It 
is  also  possible,  in  case  of  a  very  bright  but  undulating  image,  that  the  sharp  cusps  may 
from  time  to  time  be  momentarily  brought  together  by  atmospheric  undulations  before 
contact  really  occurs.  These  are  points  upon  which  the  observer  must  be  left  to  his  own 
judgment.  He  must  in  all  cases  try  to  estimate  what  the  phenomenon  would  be  if  there 
were  no  undulation^,  and  he  will  be  assisted  in  this  by  taking  particular  note  of  the 
appearance  at  those  moments  of  steadiness  which  generally  occur  every  few  seconds  in 
the  worst  atmosphere.  The  trouble  to  which  observers  are  prone  is  that  of  catching 
some  phenomenon  or  undulation,  which  really  occurs  only  from  time  to  time,  and 
fixing  the  attention  on  it  as  though  it  were  permanent.  It  is  so  easy  to  imagine  that 
one  sees  irregular  phenomena  that  the  observer  must  be  especially  careful  to  distin- 
guish what  is  permanent  from  what  is  an  accidental  product  of  atmospheric  vibration. 

The  preceding  directions  apply  principally  to  those  cases  in  which  the  air  is  clear 
and  the  sky  blue.  If  the  observation  is  made  through  a  sky  so  covered  by  clouds  or 
haze  that  there  is  no  striking  contrast  between  the  brilliancy  of  the  sun  and  that  of  the 
surrounding  sky,  the  observation  may  be  extremely  difficult,  because  the  completion 
of  the  thread  of  light  will  probably  not  be  seen  until  a  considerable  period  after 
actual  contact.  It  is  therefore  best  in  such  cases  that  the  observer  should  note  the  last 
moment  at  which  he  felt  sure  the  limbs  did  not  become  tangent  and  the  first  moment 
at  which  it  became  permanently  evident  that  the  planet  had  passed  entirely  within  the 
sun.  Perhaps  no  better  definition  can  be  given  of  contact  under  such  circumstances 
than  that  it  is  the  moment  when  the  limbs  are  really  tangent. 

Whatever  moment  the  observer  may  note,  it  is  indispensable  that  he  give  an  accu- 
rate statement  of  the  appearance  presented  by  the  sun  and  planet  at  that  moment, 
accompanied  by  a  drawing  if  necessary.  If  he  is  able  also  to  give  a  drawing  or 
description  for  the  moment  at  which  he  last  spoke  the  words  "not  yet,"  and  at  which 
he  first  said  "past,"  it  will  be  well  to  do  so.  At  the  same  time  the  useless  multiplica- 
tion of  times  is  to  be  guarded  against,  owing  to  the  distraction  thus  produced. 

Egress. — At  egress  the  phases  occur  in  inverse  order,  so  that  the  same  directions 
will  apply  when  properly  interpreted  with  the  respect  to  time.  The  following  points 
are,  however,  to  be  especially  noted: 

As  the  thread  of  light  between  the  limbs  of  the  sun  and  planet  becomes  very  thin 
it  will  probably  appear  to  darken,  partly  from  atmospheric  irradiation  and  partly  from 
the  eye  being  less  affected  by  a  thin  line  than  by  a  broad  band  of  equal  brilliancy. 
If  the  atmosphere  is  undulating  this  thread  may  be  expected  to  break  up  intb  a  mass 
of  undulating  threads  of  light,  continually  changing  their  form  and  appearance.  So 
long  as  this  undulating  mass  continues  to  be  seen  in  motion  all  the  way  across  the 
dark  space  near  the  point  of  contact,  so  long  contact  has  not  occurred.  If,  however, 
the  appearance  of  congealing  into  a  hard  immovable  mass  is  presented,  the  moment 
of  such  seeming  congelation  is  that  of  true  contact. 

In  looking  for  the  complete  disappearance  of  the  undulating  light  the  observer 
must  be  011  his  guard  against  mistaking  the  illumination  of  the  outline  of  Venus,  pro- 
duced by  the  atmosphere  of  the  planet,  for  the  true  light  of  the  sun's  limb.  There 
were  supposed  to  be  some  cases  during  the  transit  of  1874  in  which  the  observer, 


44  INSTRUCTIONS  FOR  OBSERVING  THE 

watching  for  the  fading  line  of  light  to  disappear,  found  himself  really  observing  this 
atmospheric,  outline  after  contact  was  past.  This  is  a  point  on  which,  in  the  absence 
of  complete  experience,  no  definite  instruction  can  be.  given  to  the  observer,  and  he 
must  rely  upon  his  own  judgment  to  guard  against  a  mistake  of  this  kind. 

In  the  case  of  parties  supplied  with  double-image  micrometers  it  is  recommended 
that  measures  of  the  thickness  of  the  band  of  light  between  the-  two  limbs  be  com- 
menced as  soon  after  internal  contact  as  the  observer  has  made  all  his  necessary  records 
and  notes.  It  will  also  be  well,  twenty  minutes  before  second  internal  contact,  to  com- 
mence similar  measurements  of  the  thickness  of  the  point  of  light  between  the  linil>s. 
Great  care  must,  however,  be  taken  to  stop  these  measures  and  replace  the  micrometer 
by  the  eye-piece  in  good  time  to  make  a  careful  observation  of  contact. 

Last  contact. — The  observer  should  note  the  last  moment  at  which  he  certainly 
and  distinctly  saw  the  vanishing-notch  made  by  the  receding  planet.  To  assist  in 
this  it  will  be  well  to  pronounce  the  word  "notch"  from  time  to  time  and  have  the 
times  noted  by  the  assistant. 

XXXVII.— METHODS  OF  RECORDING  CONTACTS. 

To  make  the  best  possible  observations  of  contacts  the  observer  must  be  well 
prepared  to  note  the  times  of  such  phenomena  as  he  may  see,  and  this  without  any 
liability  to  errors  of  10,  20,  or  80  seconds,  or  a  whole  minute.  If  he  has  to  look  at 
and  read  a  time-piece  himself  there  is  danger  of  such  errors.  They  may  be  avoided 
by  employing  a  chronograph,  but  in  observing  contacts  there  are  two  difficulties  con- 
nected with  the  use  of  this  instrument.  The  first  is  that  in  the  event  of  other  signals 
than  those  of  contact  being  made,  whether  by  accident  or  design,  it  may  be  difficult 
to  recognize  the  meaning  of  the  several  signals.  The  second  is  that  in  general  the 
instant  of  contact  can  be  recognized  only  by  watching  the  course  of  the  phenomena 
both  before  and  after  that  event,  and  thus  the  observer  is  not  ready  to  record  the  con- 
tact till  some  seconds  after  it  has  occurred.  Still,  with  proper  precautions  against 
these  difficulties,  a  chronograph  may  be  used. 

Experience  has  shown  that  when  an  observer  notes  the  times  himself,  the  surest 
way  of  guarding  against  errors  in  the  seconds  is  to  have  an  assistant  at  the  clock  or 
chronometer  to  beat  every  second  with  a  key,  or  small  hammer,  upon  a  board.  By 
this  plan  a  pocket  watch  may  be  used  in  the  absence  of  a  better  time-piece.  At  the 
moment  of  each  beat  the  assistant  must  call  out  only  the  units  or  the  tens  of  the  sec- 
ond. Thus,  beginning  at  ten  seconds,  the  calls  will  be  ten,  one,  two,  three,  etc., 
twenty,  one,  two,  etc..  thirty,  one,  two,  etc.  The  reason  for  not  calling  the  numbers 
in  full,  twenty-one,  twenty-two,  etc.,  is  that  their  distinct  pronunciation  would  require 
such  a  considerable  portion  of  a  second  that  the  observer  might  be  in  doubt  which 
beat  anyone  number  belonged  to.  The  simple  numerals  from  one  to  ten  may  be 
pronounced  simultaneously  with  the  beats,  so  as  to  leave  no  doubt.  An  assistant 
beating  in  this  way  may  give  time  to  several  persons. 

If  the  observer  employs  an  assistant  at  the  time-piece  to  read  off  and  record  his 
times,  he  must  also  arrange  beforehand  a  system  for  registering  notes  respecting  the 
p'"' neiia.  Such  notes  will  be  "not  yet,"  "haziness  near  the  point  of  contact," 


TEANSIT  OF  VENUS,  DECEMBER  6,  1882. 


45 


"flashes  around  the  planet  before  contact,"  "atmosphere  of  Venus  clearly  illuminated," 
etc.  If  several  such  notes  have  to  be  made,  two  assistants  will  be  necessary — one  to 
write  them  down  and  the  other  to  record  the  times.  To  co-ordinate  the  notes  with 
the  times,  the  letters  A,  B,  C,  etc.,  may  be  employed.  The  assistant  at  the  chronome- 
ter is  then  to  record  the  chronometer  time  at  which  the  letter  was  spoken,  and  opposite 
it  the  letter  itself,  while  the  other  assistant  is  to  write  down  first  the  letter  and  then 
the  note. 

XXXVIII.— DIAMETEE  OF  VENUS. 

If  the  chief  of  party  can  spare  any  time  from  the  photographic  operations 
between  second  and  third  contact,  it  should  be  employed  in  measuring  the  diameter  of 
Venus  with  the  double-image  micrometer.  Such  measures  may  be  made  in  groups  of 
sixteen,  viz :  Foiir  measures  of  the  polar  diameter,  two  of  them  being  made  with  the 
index  to  the  right  of  zero,  and  two  with  it  to  the  left ;  eight  measures  of  the  equato- 
rial diameter,  four  being  with  index  to  right,  and  four  with  index  to  left ;  and 
lastly,  four  more  measures  of  the  polar  diameter,  two  being  with  index  to  right,  and 
two  with  index  to  left.  By  this  arrangement  the  zero  point  and  all  errors  symmetrical 
with  the  time  are  eliminated.  FORM  II  contains  some  observations  of  the  diameter 
of  Mercury,  made  during  its  transit  in  May,  1878,  at  Austin,  Texas,  which  are  given 
as  a  specimen  of  such  work.  The  numbers  in  the  first  column  are,  the  chronometer 
time  when  the  measurements  were  begun,  namely  9.14  =  9b  14™;  the  reading  of  an 
aneroid  barometer,  28.98  inches  ;  the  temperature  of  the  air,  92°  Fahrenheit ;  and  the 
chronometer  time  when  the  measures  were  finished,  9.30  =  gh  30"':  The  other  columns 
are  sufficiently  explained  by  their  headings.  One  revolution  of  the  micrometer-screw 
is  equivalent  to  i  7". 208,  and  the  diameters  are  the  product  of  the  sums  by  one-quarter 
of  this  value. 

FORM  II. 


Time, 
Barom. 
•  Ther. 

Diameter 
Measured. 

Readings  of  Screw. 

Differences. 

To  Left. 

To  Right. 

Polar. 

Equatorial. 

9.14 

Polar   .    .    . 

r. 
14.91 

.91 

14.91 
.92 

•93 
.92 

14.91 
.91 

r 
"3-Si 
•51 

I3-5I 
•54 

•53 
•  52 

13-51 

•52 

r. 
1.40 

1.40 

1.40 
1-39 

r. 

28.98 
92.0 

Equatorial  . 

1.40 

•38 
.40 
1.40 

9-3° 

Polar   .    .    . 

Sums         

5-59 
24"-°5 

5.58 

Diameter  of  Planet    .... 

24".  co 

46  INSTRUCTIONS  FOR  OBSERVING  THE 

• 

\\XIX.-DATA  REQUIRED. 

For  convenience  of  reference,  the  data  which  will  be  required  in  reducing  the 
observations  an-  hero  enumerated.  D  3,  4,  5,  6,  7,  G  3,  6,  8,  9,  10,  II  3,  and  K  4 
can  be  best  determined  in  Washington.  All  the  others  must  be  determined  in  the  field, 
and  it  will  be  the  special  duty  of  the  chief  of  party  to  see  that  nothing  needful  is 
omitted  from  the  record. 

A. — Name  of  station. 

B  — Latitude  and  longitude  of  station. 

C. — With  every  observation,  the  name  of  each  person  employed  in  making  it,  and  the 

part  he  took  in  the  work,  must  be  recorded. 
D — For  the  meridian  instrument: 

1.  Maker's  name  and  number. 

2.  Size  and  power  of  telescope. 

3.  Correction  for  flexure. 

4.  Correction  for  inequality  of  pivots. 

5.  Value  of  scale  of  striding  level. 

6.  Value  of  scale  of  zenith  distance  level. 

7.  Value  of  one  revolution  of  the  micrometer  screw. 

8.  Intervals  of  transit  wires. 

9.  Intervals  of  micrometer  wires. 
K. — For  the  chronometers: 

1.  Makers'  names,  numbers,  and  descriptions,  whether  mean  time  or  sidereal,  num 

ber  of  hours  on  dial,  break-circuit  or  not,  etc. 

2.  The  correction  of  each  chronometer  to  local  time  whenever  it  is  used. 
F. — The  maker's  name,  and  number  of  the  chronograph. 

G. — For  the  photoheliograph: 

1 .  Number  of  objective. 

2.  Number  of  measuring-rod. 

3.  Length  of  measuring-rod  at  some  definite  temperature. 

4.  Temperature  of  measuring-rod  whenever  it  is  used. 

5.  Number  of  jaw-micrometer. 

6.  Correction  to  be  applied  to  readings  of  jaw-micrometer. 

7.  Distance  from  back  surface  of  objective  to  front  surface  of  reticule-plate. 

8.  Distance  from  back  surface  of  the  objective  to  its  second  principal  point. 

9.  Thickness  of  reticule-plate. 

10.  Refractive  index  of  reticule-plate. 

11.  Interval  between  reticule-plate  and  collodion-film. 

12.  A/imuth  of  the  center  of  the  reticule-plate. 

13.  Level,  or  zenith  distance,  of  the  center  of  the  reticule-plate. 

14.  When  the  instrument  is  in   use.  the  temperature  of  the  atmosphere  in  the 

shade,  the  temperature  in  die  photographic  house,  and  the  reading  of  the 
barometer  and  its  attached  thermometer,  must  be  recorded  every  half 
hour. 


TRANSIT  OF  VENUS,  DECEMBER  6,  1882.  47 

G. — For  the  photoheliograph — Continued. 

15.  Every  negative  must  have  its  number  marked  upon  it  by  a  diamond.  With 
each  negative  must  be  recorded  the  chronometer  time  of  its  exposure,  a 
sidereal  chronometer  being  used;  the  direction  (east  or  west)  of  the  small 
arm  on  top  of  the  frame,  from  the  center  of  which  the  plumb-line  is 
suspended ;  and  the  direction  of  motion  of  the  exposing-slide  (east  or  west). 

H. — For  the  engineer's  level,  or  level  of  precision : 

1.  Maker's  name  and  number. 

2.  Size  and  power  of  telescope. 

3.  Value  of  level-scale. 

4.  Value  of  micrometer-screw. 
I. — For  the  theodolite: 

1.  Maker's  name  and  number. 

2.  Size  and  power  of  telescope. 

3.  Diameters  of  limbs,  and  least  reading. 
K. — For  the  equatorial  telescope: 

1.  Maker's  name  and  number 

2.  Size  of  telescope. 

3.  Powers  of  eye-pieces. 

4.  Value  of  one  revolution  of  the  screw  of  the  double-image  micrometer  for  each 

of  the  two  front  lenses. 

5.  With  every  observation,  the  power  employed  in  making  it  must  be  recorded. 

The  utmost  care  must  be  taken  to  have  accurate  knowledge  of  the  local  time  on 
the  day  of  the  transit,  Owing  to  the  uncertainty  of  weather,  no  fair  night  must 
be  allowed  to  pass  during  the  week  preceding  December  6  without  the  observation  of 
star-transits  for  time  and  azimuth ;  but  if  clouds  prevent  sucli  observations,  then,  if 
possible,  the  transit  of  both  limbs  of  the  sun  must  be  observed  daily  during  the  same 
period,  the  telescope  being  reversed  between  the  limbs.  On  the  day  of  the  transit  all  the 
chronometers  at  the  station  must  be  compared  in  the  morning,  and  again  in  the  evening. 

On  December  5th  all  the  apparatus  must  be  inspected  to  make  sure  that  it  is  in 
good  working  order,  and  care  must  be  taken  that  the  adjustments  4  and  5  of  section 
V  are  correct.  Both  on  the  day  of  the  transit  and  on  the  day  preceding,  G  7  and 
G  13  must  be  very  accurately  determined;  and  G  12  must  be  deduced  from  the  tran- 
sits of  stars  observed  on  nights  before  and  after  the  transit  of  Venus,  but  as  near  to 
that  event  as  possible. 

Examples  showing  how  observations  of  G  13  should  be  made  and  recorded  are 
appended.  In  explanation  of  them  it  is  only  necessary  to  say  that  FORM  III  refers  to 
observations  with  an  engineer's  level,  and  FORM  IV  to  observations  with  a  level  of  pre- 
cision. The  theory  of  these  observations  has  been  already  given  in  section  V.  That 
end  of  the  bubble  which  gives  the  largest  reading  is  toward  the  high  end  of  the  line, 
and  the  amount  of  elevation  is  found  by  multiplying  the  number  in  the  line  "Differ- 
ence" into  one-sixteenth  of  the  value  of  one  division  of  the  level  scale.  For  the  level 
Stackpole  and  Brother,  No.  1510,  the  valve  of  one  division  was  6". 54.  One-sixteenth 
of  this  is  o".4O9,  which  multiplied  by  9.8  divisions  gives  4".oi.  For  the  level 


48 


INSTRUCTIONS  FOR  OBSERVING  THE 


Stackpole  and  Brother,  No.  1489,  the  value  of  one  division  is  i".74,  one-sixteenth  of 
this  is  o".iO9,  which  multiplied  by  22.0  divisions  gives  2".4O.  The  large  collimation 
error  of  the  telescope  of  this  level  is  noticeable. 

FORM  III. — Observations  made  with  the  engineer's  level  STACKPOLE  and  BROTHER,  No. 
1510,  to  deter  mitir  the  inclination  of  the  line  of  collimation  of  the  horizontal  jilidlo'icl  la- 
graph  at  Hobart  Town,  Tasmania,  December  9,  1874. 


Object 
Observed. 

End  of  Bubble. 

North. 

South. 

North. 

South. 

Reticule    . 

d. 
45-0 

d. 
'3-9 

d. 

d. 

44-6 

13-9 

•      • 

Transit 

• 

. 

12.  0 

43-0 

•      • 

•     • 

12.5 

43-4 

•      • 

•     • 

12.2 

43-3 

•     • 

•     • 

12.2 

43.0 

Reticule    . 

44.0 

13.0 

. 

44-0 

13.0 

•       • 

•     • 

Sums  . 

177.6 

53-8 

48.  y 

172.7 

Sums  .     . 

23'-4 

221  .6 

Difference 

9.8 

North  end  high,  4".or 

FORM  IV. — Observations  made  with,  the  level  of  precision,  STACKPOLE  and  BROTHER,  No. 
1489,  to  determine  Hie  inclination  of  the  line  of  collimation  of  the  horizontal  jihotohclio- 
graph  at  Washington,  D.  C.,  September  g,  1882. 


Focusing 
Pinion. 

tnd  of 
Bubble. 

Level 
Direct. 

Level 
Reversed. 

Right      . 

N. 

23-5 

44-0 

S. 

51-5 

15.0 

Lelt    .      . 

N. 

13-0 

54-0 

S. 

42.0 

25-5 

N. 

12.  0 

55-0 

S. 

4I.O 

26.0 

Right       . 

N. 

23.O 

44-5 

S. 

51-5 

15-5 

Sums     .... 

257-5 

2/9-5 

Difference  . 

22.0 

North  end  high,  2".4O 

TRANSIT  OF  VENUS,  DECEMBER  6,  !882.  49 

XL.— RECORDS  OF  OBSERVATIONS  AND  OPERATIONS. 

Of  the  journals  and  memorandum-books,  each  observing-room  is  to  have  at  least 
one  for  its  exclusive  use,  in  which  every  operation  must  be  recorded  in  detail,  with 
all  the  particulars  necessary  to  its  being  full}-  understood.  A  journal  is  also  to  be 
kept,  in  which  all  the  operations  of  the  party  must  be  entered. 

In  addition  to  the  original  record  of  the  observations,  a  duplicate  record  must  be 
made  with  ink,  at  the  earliest  possible  moment.  The  following  rules  respecting  the  cor- 
rection of  supposed  mistakes  must  be  attended  to  in  the  case  of  each  set  of  records: 

Original  rouc/lt  record. — In  this  record  a  number  once  written  should  never  be 
erased.  If  the  observer  detects  a  wrong  number  immediately  after  writing  it,  he  must 
draw  a  line  through  it  and  write  the  correct  number  beside  it.  If  it  is  concluded 
from  subsequent  observations  that  a  number  is  probably  wrong,  that  fact  must  be  noted, 
and  the  correct  number  indicated ;  but  the  record  as  written  must  not  be  altered.  Num- 
bers should  not  be  inserted  in  this  record  which  are  the  result  of  calculation.  For 
instance,  if  the  observer  fails  to  note  the  minutes  of  the  chronometer  corresponding  to 
any  observation,  he  must  not  conclude  what  they  were  from  the  preceding  or  subse- 
quent observations,  and  then  put  them  in,  but  must  omit  them  entirely,  unless  such 
omission  would  cause  subsequent  uncertainty.  In  that  case  the  necessary  numbers 
may  be  inserted,  provided  it  is  done  in  such  a  manner  as  to  show  that  they  were  not 
directly  observed,  but  are  concluded  from  other  parts  of  the  record.  To  indicate  this, 
a  circle  may  be  drawn  around  them. 

Should  it  be  found  necessary  from  any  cause  whatever  to  make  the  first  rough 
notes  of  an  observation  upon  loose  slips  of  paper,  these  slips  must  be  carefully  pasted 
into  the  proper  note  book,  in  immediate  proximity  to  the  formal  record  which  has  been 
copied  from  them. 

The  duplicate  or  fair  copy. — The  second  record  is  to  be  copied  from  the  first  as 
soon  as  practicable  after  making  the  observations,  so  that  if  mistakes  exist  they  may 
be  detected  and  corrected.  If  blank  forms  for  the  observations  are  provided,  they 
may  be  used  for  the  second  copy  as  well  as  for  the  first;  but  everything  for  which 
they  are  not  available  must  be  copied  in  chronological  order  into  a  single  book.  In 
this  duplicate  copy  greater  liberty  will  be  allowed  respecting  additions  and  alterations 
than  in  the  original,  the  object  being  to  make  a  complete  and  correct  record;  but  in  the 
event  of  numbers  being  added  as  the  result  of  calculation,  they  should  lie  underscored 
with  red  ink,  or  otherwise  indicated. 

All  records  of  observations  must  include  every  particular  necessary  to  their  being 
completely  understood.  For  instance,  wherever  time  is  given,  the  particular  time- 
piece employed  must  be  designated;  where  level  readings  are  given,  the  direction  of 
each  end  of  the  level,  east,  west,  north,  or  south,  must  be  recorded;  when  the  images 
of  lines  on  the  plate-holder  are  observed  with  the  transit,  the  direction  of  the  image 
from  the  middle  wire,  whether  right,  left,  north,  or  south,  must  be  stated,  as  well  as  the 
direction  of  the  eye-piece  of  the  instrument,  east  or  west;  and  all  photographs  of  the 
sun  must  indicate  the  exact  time  at  which  they  were  taken,  and  must  be  so  marked 
that  the  position  of  the  plate  Mien  in  the  holder — that  is,  the  top,  bottom,  east  and 
west  sides — can  always  be  distinguished. 

7   INS 


50  INSTRUCTIONS  FOK  OBSK1!VIN<!  THK  TK'ANSIT  OF  YKNUS. 

In  using  the  chronograph,  the  minutes  .and  seconds  must  always  be  marked  upon 
the  sheet  at  least  twice  during  each  series  of  observations.  All  chronograph  sheets 
must  be  preserved  and  sent  home  with  the  other  original  records,  but  in  addition  to 
this  they  must  also  be  read  off  and  recorded  in  the  proper  books. 

XLL— TRANSMISSION  AND  PUBLICATION  OF  OBSERVATIONS. 

All  members  of  the  parties  are  prohibited  from  publishing  their  observations,  or 
results,  without  authority  of  the  commission;  but  this  prohibition  is  not  intended  to 
prevent  any  general  statements  respecting  the  nature  and  success  of  the  work  which 
the  observers  may  choose  to  make.  In  cases  of  co-operation  with  any  other  individual 
or  party,  the  chief  of  the  party  is  authorized  to  communicate  to  the  other  copies  of 
all  observations  necessary  to  the  special  end  for  which  the  co-operation  was  entered 
into. 

The  chief  of  each  party  will  transmit  all  the  records  to  the  president  of  the  com- 
mission as  soon  as  practicable  after  the  completion  of  the  observations.  They  will  be 
sent  in  separate  packets,  one  containing  the  journal  and  all  the  original  pencil  memo- 
randa of  the  observations,  the  other  the  fair  copy  already  directed  to  be  made.  If 
practicable,  the  two  packets  must  be  sent  at  different  times  and  by  different  convey- 
ances. From  ports  where  an  American  consul  is  stationed,  thev  may  be  forwarded 
by  him  through  the  Department  of  State,  in  which  case  he  must  be  notified  to  send 
the  two  packets  by  different  ships. 


;-   tr  -.   : 
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"<•  ?  •'- 


